![Peter Zijlstra](/assets/img/avatar_default.png)
Extend lockdep to validate lock wait-type context. The current wait-types are: LD_WAIT_FREE, /* wait free, rcu etc.. */ LD_WAIT_SPIN, /* spin loops, raw_spinlock_t etc.. */ LD_WAIT_CONFIG, /* CONFIG_PREEMPT_LOCK, spinlock_t etc.. */ LD_WAIT_SLEEP, /* sleeping locks, mutex_t etc.. */ Where lockdep validates that the current lock (the one being acquired) fits in the current wait-context (as generated by the held stack). This ensures that there is no attempt to acquire mutexes while holding spinlocks, to acquire spinlocks while holding raw_spinlocks and so on. In other words, its a more fancy might_sleep(). Obviously RCU made the entire ordeal more complex than a simple single value test because RCU can be acquired in (pretty much) any context and while it presents a context to nested locks it is not the same as it got acquired in. Therefore its necessary to split the wait_type into two values, one representing the acquire (outer) and one representing the nested context (inner). For most 'normal' locks these two are the same. [ To make static initialization easier we have the rule that: .outer == INV means .outer == .inner; because INV == 0. ] It further means that its required to find the minimal .inner of the held stack to compare against the outer of the new lock; because while 'normal' RCU presents a CONFIG type to nested locks, if it is taken while already holding a SPIN type it obviously doesn't relax the rules. Below is an example output generated by the trivial test code: raw_spin_lock(&foo); spin_lock(&bar); spin_unlock(&bar); raw_spin_unlock(&foo); [ BUG: Invalid wait context ] ----------------------------- swapper/0/1 is trying to lock: ffffc90000013f20 (&bar){....}-{3:3}, at: kernel_init+0xdb/0x187 other info that might help us debug this: 1 lock held by swapper/0/1: #0: ffffc90000013ee0 (&foo){+.+.}-{2:2}, at: kernel_init+0xd1/0x187 The way to read it is to look at the new -{n,m} part in the lock description; -{3:3} for the attempted lock, and try and match that up to the held locks, which in this case is the one: -{2,2}. This tells that the acquiring lock requires a more relaxed environment than presented by the lock stack. Currently only the normal locks and RCU are converted, the rest of the lockdep users defaults to .inner = INV which is ignored. More conversions can be done when desired. The check for spinlock_t nesting is not enabled by default. It's a separate config option for now as there are known problems which are currently addressed. The config option allows to identify these problems and to verify that the solutions found are indeed solving them. The config switch will be removed and the checks will permanently enabled once the vast majority of issues has been addressed. [ bigeasy: Move LD_WAIT_FREE,… out of CONFIG_LOCKDEP to avoid compile failure with CONFIG_DEBUG_SPINLOCK + !CONFIG_LOCKDEP] [ tglx: Add the config option ] Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20200321113242.427089655@linutronix.de
1655 lines
47 KiB
C
1655 lines
47 KiB
C
// SPDX-License-Identifier: GPL-2.0
|
|
/* kernel/rwsem.c: R/W semaphores, public implementation
|
|
*
|
|
* Written by David Howells (dhowells@redhat.com).
|
|
* Derived from asm-i386/semaphore.h
|
|
*
|
|
* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
|
|
* and Michel Lespinasse <walken@google.com>
|
|
*
|
|
* Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
|
|
* and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
|
|
*
|
|
* Rwsem count bit fields re-definition and rwsem rearchitecture by
|
|
* Waiman Long <longman@redhat.com> and
|
|
* Peter Zijlstra <peterz@infradead.org>.
|
|
*/
|
|
|
|
#include <linux/types.h>
|
|
#include <linux/kernel.h>
|
|
#include <linux/sched.h>
|
|
#include <linux/sched/rt.h>
|
|
#include <linux/sched/task.h>
|
|
#include <linux/sched/debug.h>
|
|
#include <linux/sched/wake_q.h>
|
|
#include <linux/sched/signal.h>
|
|
#include <linux/sched/clock.h>
|
|
#include <linux/export.h>
|
|
#include <linux/rwsem.h>
|
|
#include <linux/atomic.h>
|
|
|
|
#include "lock_events.h"
|
|
|
|
/*
|
|
* The least significant 3 bits of the owner value has the following
|
|
* meanings when set.
|
|
* - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers
|
|
* - Bit 1: RWSEM_RD_NONSPINNABLE - Readers cannot spin on this lock.
|
|
* - Bit 2: RWSEM_WR_NONSPINNABLE - Writers cannot spin on this lock.
|
|
*
|
|
* When the rwsem is either owned by an anonymous writer, or it is
|
|
* reader-owned, but a spinning writer has timed out, both nonspinnable
|
|
* bits will be set to disable optimistic spinning by readers and writers.
|
|
* In the later case, the last unlocking reader should then check the
|
|
* writer nonspinnable bit and clear it only to give writers preference
|
|
* to acquire the lock via optimistic spinning, but not readers. Similar
|
|
* action is also done in the reader slowpath.
|
|
|
|
* When a writer acquires a rwsem, it puts its task_struct pointer
|
|
* into the owner field. It is cleared after an unlock.
|
|
*
|
|
* When a reader acquires a rwsem, it will also puts its task_struct
|
|
* pointer into the owner field with the RWSEM_READER_OWNED bit set.
|
|
* On unlock, the owner field will largely be left untouched. So
|
|
* for a free or reader-owned rwsem, the owner value may contain
|
|
* information about the last reader that acquires the rwsem.
|
|
*
|
|
* That information may be helpful in debugging cases where the system
|
|
* seems to hang on a reader owned rwsem especially if only one reader
|
|
* is involved. Ideally we would like to track all the readers that own
|
|
* a rwsem, but the overhead is simply too big.
|
|
*
|
|
* Reader optimistic spinning is helpful when the reader critical section
|
|
* is short and there aren't that many readers around. It makes readers
|
|
* relatively more preferred than writers. When a writer times out spinning
|
|
* on a reader-owned lock and set the nospinnable bits, there are two main
|
|
* reasons for that.
|
|
*
|
|
* 1) The reader critical section is long, perhaps the task sleeps after
|
|
* acquiring the read lock.
|
|
* 2) There are just too many readers contending the lock causing it to
|
|
* take a while to service all of them.
|
|
*
|
|
* In the former case, long reader critical section will impede the progress
|
|
* of writers which is usually more important for system performance. In
|
|
* the later case, reader optimistic spinning tends to make the reader
|
|
* groups that contain readers that acquire the lock together smaller
|
|
* leading to more of them. That may hurt performance in some cases. In
|
|
* other words, the setting of nonspinnable bits indicates that reader
|
|
* optimistic spinning may not be helpful for those workloads that cause
|
|
* it.
|
|
*
|
|
* Therefore, any writers that had observed the setting of the writer
|
|
* nonspinnable bit for a given rwsem after they fail to acquire the lock
|
|
* via optimistic spinning will set the reader nonspinnable bit once they
|
|
* acquire the write lock. Similarly, readers that observe the setting
|
|
* of reader nonspinnable bit at slowpath entry will set the reader
|
|
* nonspinnable bits when they acquire the read lock via the wakeup path.
|
|
*
|
|
* Once the reader nonspinnable bit is on, it will only be reset when
|
|
* a writer is able to acquire the rwsem in the fast path or somehow a
|
|
* reader or writer in the slowpath doesn't observe the nonspinable bit.
|
|
*
|
|
* This is to discourage reader optmistic spinning on that particular
|
|
* rwsem and make writers more preferred. This adaptive disabling of reader
|
|
* optimistic spinning will alleviate the negative side effect of this
|
|
* feature.
|
|
*/
|
|
#define RWSEM_READER_OWNED (1UL << 0)
|
|
#define RWSEM_RD_NONSPINNABLE (1UL << 1)
|
|
#define RWSEM_WR_NONSPINNABLE (1UL << 2)
|
|
#define RWSEM_NONSPINNABLE (RWSEM_RD_NONSPINNABLE | RWSEM_WR_NONSPINNABLE)
|
|
#define RWSEM_OWNER_FLAGS_MASK (RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)
|
|
|
|
#ifdef CONFIG_DEBUG_RWSEMS
|
|
# define DEBUG_RWSEMS_WARN_ON(c, sem) do { \
|
|
if (!debug_locks_silent && \
|
|
WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
|
|
#c, atomic_long_read(&(sem)->count), \
|
|
(unsigned long) sem->magic, \
|
|
atomic_long_read(&(sem)->owner), (long)current, \
|
|
list_empty(&(sem)->wait_list) ? "" : "not ")) \
|
|
debug_locks_off(); \
|
|
} while (0)
|
|
#else
|
|
# define DEBUG_RWSEMS_WARN_ON(c, sem)
|
|
#endif
|
|
|
|
/*
|
|
* On 64-bit architectures, the bit definitions of the count are:
|
|
*
|
|
* Bit 0 - writer locked bit
|
|
* Bit 1 - waiters present bit
|
|
* Bit 2 - lock handoff bit
|
|
* Bits 3-7 - reserved
|
|
* Bits 8-62 - 55-bit reader count
|
|
* Bit 63 - read fail bit
|
|
*
|
|
* On 32-bit architectures, the bit definitions of the count are:
|
|
*
|
|
* Bit 0 - writer locked bit
|
|
* Bit 1 - waiters present bit
|
|
* Bit 2 - lock handoff bit
|
|
* Bits 3-7 - reserved
|
|
* Bits 8-30 - 23-bit reader count
|
|
* Bit 31 - read fail bit
|
|
*
|
|
* It is not likely that the most significant bit (read fail bit) will ever
|
|
* be set. This guard bit is still checked anyway in the down_read() fastpath
|
|
* just in case we need to use up more of the reader bits for other purpose
|
|
* in the future.
|
|
*
|
|
* atomic_long_fetch_add() is used to obtain reader lock, whereas
|
|
* atomic_long_cmpxchg() will be used to obtain writer lock.
|
|
*
|
|
* There are three places where the lock handoff bit may be set or cleared.
|
|
* 1) rwsem_mark_wake() for readers.
|
|
* 2) rwsem_try_write_lock() for writers.
|
|
* 3) Error path of rwsem_down_write_slowpath().
|
|
*
|
|
* For all the above cases, wait_lock will be held. A writer must also
|
|
* be the first one in the wait_list to be eligible for setting the handoff
|
|
* bit. So concurrent setting/clearing of handoff bit is not possible.
|
|
*/
|
|
#define RWSEM_WRITER_LOCKED (1UL << 0)
|
|
#define RWSEM_FLAG_WAITERS (1UL << 1)
|
|
#define RWSEM_FLAG_HANDOFF (1UL << 2)
|
|
#define RWSEM_FLAG_READFAIL (1UL << (BITS_PER_LONG - 1))
|
|
|
|
#define RWSEM_READER_SHIFT 8
|
|
#define RWSEM_READER_BIAS (1UL << RWSEM_READER_SHIFT)
|
|
#define RWSEM_READER_MASK (~(RWSEM_READER_BIAS - 1))
|
|
#define RWSEM_WRITER_MASK RWSEM_WRITER_LOCKED
|
|
#define RWSEM_LOCK_MASK (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
|
|
#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
|
|
RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)
|
|
|
|
/*
|
|
* All writes to owner are protected by WRITE_ONCE() to make sure that
|
|
* store tearing can't happen as optimistic spinners may read and use
|
|
* the owner value concurrently without lock. Read from owner, however,
|
|
* may not need READ_ONCE() as long as the pointer value is only used
|
|
* for comparison and isn't being dereferenced.
|
|
*/
|
|
static inline void rwsem_set_owner(struct rw_semaphore *sem)
|
|
{
|
|
atomic_long_set(&sem->owner, (long)current);
|
|
}
|
|
|
|
static inline void rwsem_clear_owner(struct rw_semaphore *sem)
|
|
{
|
|
atomic_long_set(&sem->owner, 0);
|
|
}
|
|
|
|
/*
|
|
* Test the flags in the owner field.
|
|
*/
|
|
static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags)
|
|
{
|
|
return atomic_long_read(&sem->owner) & flags;
|
|
}
|
|
|
|
/*
|
|
* The task_struct pointer of the last owning reader will be left in
|
|
* the owner field.
|
|
*
|
|
* Note that the owner value just indicates the task has owned the rwsem
|
|
* previously, it may not be the real owner or one of the real owners
|
|
* anymore when that field is examined, so take it with a grain of salt.
|
|
*
|
|
* The reader non-spinnable bit is preserved.
|
|
*/
|
|
static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
|
|
struct task_struct *owner)
|
|
{
|
|
unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
|
|
(atomic_long_read(&sem->owner) & RWSEM_RD_NONSPINNABLE);
|
|
|
|
atomic_long_set(&sem->owner, val);
|
|
}
|
|
|
|
static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
|
|
{
|
|
__rwsem_set_reader_owned(sem, current);
|
|
}
|
|
|
|
/*
|
|
* Return true if the rwsem is owned by a reader.
|
|
*/
|
|
static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
|
|
{
|
|
#ifdef CONFIG_DEBUG_RWSEMS
|
|
/*
|
|
* Check the count to see if it is write-locked.
|
|
*/
|
|
long count = atomic_long_read(&sem->count);
|
|
|
|
if (count & RWSEM_WRITER_MASK)
|
|
return false;
|
|
#endif
|
|
return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
|
|
}
|
|
|
|
#ifdef CONFIG_DEBUG_RWSEMS
|
|
/*
|
|
* With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
|
|
* is a task pointer in owner of a reader-owned rwsem, it will be the
|
|
* real owner or one of the real owners. The only exception is when the
|
|
* unlock is done by up_read_non_owner().
|
|
*/
|
|
static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
|
|
{
|
|
unsigned long val = atomic_long_read(&sem->owner);
|
|
|
|
while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
|
|
if (atomic_long_try_cmpxchg(&sem->owner, &val,
|
|
val & RWSEM_OWNER_FLAGS_MASK))
|
|
return;
|
|
}
|
|
}
|
|
#else
|
|
static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
|
|
{
|
|
}
|
|
#endif
|
|
|
|
/*
|
|
* Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag
|
|
* remains set. Otherwise, the operation will be aborted.
|
|
*/
|
|
static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
|
|
{
|
|
unsigned long owner = atomic_long_read(&sem->owner);
|
|
|
|
do {
|
|
if (!(owner & RWSEM_READER_OWNED))
|
|
break;
|
|
if (owner & RWSEM_NONSPINNABLE)
|
|
break;
|
|
} while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
|
|
owner | RWSEM_NONSPINNABLE));
|
|
}
|
|
|
|
static inline bool rwsem_read_trylock(struct rw_semaphore *sem)
|
|
{
|
|
long cnt = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
|
|
if (WARN_ON_ONCE(cnt < 0))
|
|
rwsem_set_nonspinnable(sem);
|
|
return !(cnt & RWSEM_READ_FAILED_MASK);
|
|
}
|
|
|
|
/*
|
|
* Return just the real task structure pointer of the owner
|
|
*/
|
|
static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
|
|
{
|
|
return (struct task_struct *)
|
|
(atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
|
|
}
|
|
|
|
/*
|
|
* Return the real task structure pointer of the owner and the embedded
|
|
* flags in the owner. pflags must be non-NULL.
|
|
*/
|
|
static inline struct task_struct *
|
|
rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
|
|
{
|
|
unsigned long owner = atomic_long_read(&sem->owner);
|
|
|
|
*pflags = owner & RWSEM_OWNER_FLAGS_MASK;
|
|
return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
|
|
}
|
|
|
|
/*
|
|
* Guide to the rw_semaphore's count field.
|
|
*
|
|
* When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
|
|
* by a writer.
|
|
*
|
|
* The lock is owned by readers when
|
|
* (1) the RWSEM_WRITER_LOCKED isn't set in count,
|
|
* (2) some of the reader bits are set in count, and
|
|
* (3) the owner field has RWSEM_READ_OWNED bit set.
|
|
*
|
|
* Having some reader bits set is not enough to guarantee a readers owned
|
|
* lock as the readers may be in the process of backing out from the count
|
|
* and a writer has just released the lock. So another writer may steal
|
|
* the lock immediately after that.
|
|
*/
|
|
|
|
/*
|
|
* Initialize an rwsem:
|
|
*/
|
|
void __init_rwsem(struct rw_semaphore *sem, const char *name,
|
|
struct lock_class_key *key)
|
|
{
|
|
#ifdef CONFIG_DEBUG_LOCK_ALLOC
|
|
/*
|
|
* Make sure we are not reinitializing a held semaphore:
|
|
*/
|
|
debug_check_no_locks_freed((void *)sem, sizeof(*sem));
|
|
lockdep_init_map_wait(&sem->dep_map, name, key, 0, LD_WAIT_SLEEP);
|
|
#endif
|
|
#ifdef CONFIG_DEBUG_RWSEMS
|
|
sem->magic = sem;
|
|
#endif
|
|
atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
|
|
raw_spin_lock_init(&sem->wait_lock);
|
|
INIT_LIST_HEAD(&sem->wait_list);
|
|
atomic_long_set(&sem->owner, 0L);
|
|
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
|
|
osq_lock_init(&sem->osq);
|
|
#endif
|
|
}
|
|
EXPORT_SYMBOL(__init_rwsem);
|
|
|
|
enum rwsem_waiter_type {
|
|
RWSEM_WAITING_FOR_WRITE,
|
|
RWSEM_WAITING_FOR_READ
|
|
};
|
|
|
|
struct rwsem_waiter {
|
|
struct list_head list;
|
|
struct task_struct *task;
|
|
enum rwsem_waiter_type type;
|
|
unsigned long timeout;
|
|
unsigned long last_rowner;
|
|
};
|
|
#define rwsem_first_waiter(sem) \
|
|
list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
|
|
|
|
enum rwsem_wake_type {
|
|
RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
|
|
RWSEM_WAKE_READERS, /* Wake readers only */
|
|
RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
|
|
};
|
|
|
|
enum writer_wait_state {
|
|
WRITER_NOT_FIRST, /* Writer is not first in wait list */
|
|
WRITER_FIRST, /* Writer is first in wait list */
|
|
WRITER_HANDOFF /* Writer is first & handoff needed */
|
|
};
|
|
|
|
/*
|
|
* The typical HZ value is either 250 or 1000. So set the minimum waiting
|
|
* time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
|
|
* queue before initiating the handoff protocol.
|
|
*/
|
|
#define RWSEM_WAIT_TIMEOUT DIV_ROUND_UP(HZ, 250)
|
|
|
|
/*
|
|
* Magic number to batch-wakeup waiting readers, even when writers are
|
|
* also present in the queue. This both limits the amount of work the
|
|
* waking thread must do and also prevents any potential counter overflow,
|
|
* however unlikely.
|
|
*/
|
|
#define MAX_READERS_WAKEUP 0x100
|
|
|
|
/*
|
|
* handle the lock release when processes blocked on it that can now run
|
|
* - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
|
|
* have been set.
|
|
* - there must be someone on the queue
|
|
* - the wait_lock must be held by the caller
|
|
* - tasks are marked for wakeup, the caller must later invoke wake_up_q()
|
|
* to actually wakeup the blocked task(s) and drop the reference count,
|
|
* preferably when the wait_lock is released
|
|
* - woken process blocks are discarded from the list after having task zeroed
|
|
* - writers are only marked woken if downgrading is false
|
|
*/
|
|
static void rwsem_mark_wake(struct rw_semaphore *sem,
|
|
enum rwsem_wake_type wake_type,
|
|
struct wake_q_head *wake_q)
|
|
{
|
|
struct rwsem_waiter *waiter, *tmp;
|
|
long oldcount, woken = 0, adjustment = 0;
|
|
struct list_head wlist;
|
|
|
|
lockdep_assert_held(&sem->wait_lock);
|
|
|
|
/*
|
|
* Take a peek at the queue head waiter such that we can determine
|
|
* the wakeup(s) to perform.
|
|
*/
|
|
waiter = rwsem_first_waiter(sem);
|
|
|
|
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
|
|
if (wake_type == RWSEM_WAKE_ANY) {
|
|
/*
|
|
* Mark writer at the front of the queue for wakeup.
|
|
* Until the task is actually later awoken later by
|
|
* the caller, other writers are able to steal it.
|
|
* Readers, on the other hand, will block as they
|
|
* will notice the queued writer.
|
|
*/
|
|
wake_q_add(wake_q, waiter->task);
|
|
lockevent_inc(rwsem_wake_writer);
|
|
}
|
|
|
|
return;
|
|
}
|
|
|
|
/*
|
|
* No reader wakeup if there are too many of them already.
|
|
*/
|
|
if (unlikely(atomic_long_read(&sem->count) < 0))
|
|
return;
|
|
|
|
/*
|
|
* Writers might steal the lock before we grant it to the next reader.
|
|
* We prefer to do the first reader grant before counting readers
|
|
* so we can bail out early if a writer stole the lock.
|
|
*/
|
|
if (wake_type != RWSEM_WAKE_READ_OWNED) {
|
|
struct task_struct *owner;
|
|
|
|
adjustment = RWSEM_READER_BIAS;
|
|
oldcount = atomic_long_fetch_add(adjustment, &sem->count);
|
|
if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
|
|
/*
|
|
* When we've been waiting "too" long (for writers
|
|
* to give up the lock), request a HANDOFF to
|
|
* force the issue.
|
|
*/
|
|
if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
|
|
time_after(jiffies, waiter->timeout)) {
|
|
adjustment -= RWSEM_FLAG_HANDOFF;
|
|
lockevent_inc(rwsem_rlock_handoff);
|
|
}
|
|
|
|
atomic_long_add(-adjustment, &sem->count);
|
|
return;
|
|
}
|
|
/*
|
|
* Set it to reader-owned to give spinners an early
|
|
* indication that readers now have the lock.
|
|
* The reader nonspinnable bit seen at slowpath entry of
|
|
* the reader is copied over.
|
|
*/
|
|
owner = waiter->task;
|
|
if (waiter->last_rowner & RWSEM_RD_NONSPINNABLE) {
|
|
owner = (void *)((unsigned long)owner | RWSEM_RD_NONSPINNABLE);
|
|
lockevent_inc(rwsem_opt_norspin);
|
|
}
|
|
__rwsem_set_reader_owned(sem, owner);
|
|
}
|
|
|
|
/*
|
|
* Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
|
|
* queue. We know that the woken will be at least 1 as we accounted
|
|
* for above. Note we increment the 'active part' of the count by the
|
|
* number of readers before waking any processes up.
|
|
*
|
|
* This is an adaptation of the phase-fair R/W locks where at the
|
|
* reader phase (first waiter is a reader), all readers are eligible
|
|
* to acquire the lock at the same time irrespective of their order
|
|
* in the queue. The writers acquire the lock according to their
|
|
* order in the queue.
|
|
*
|
|
* We have to do wakeup in 2 passes to prevent the possibility that
|
|
* the reader count may be decremented before it is incremented. It
|
|
* is because the to-be-woken waiter may not have slept yet. So it
|
|
* may see waiter->task got cleared, finish its critical section and
|
|
* do an unlock before the reader count increment.
|
|
*
|
|
* 1) Collect the read-waiters in a separate list, count them and
|
|
* fully increment the reader count in rwsem.
|
|
* 2) For each waiters in the new list, clear waiter->task and
|
|
* put them into wake_q to be woken up later.
|
|
*/
|
|
INIT_LIST_HEAD(&wlist);
|
|
list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
|
|
if (waiter->type == RWSEM_WAITING_FOR_WRITE)
|
|
continue;
|
|
|
|
woken++;
|
|
list_move_tail(&waiter->list, &wlist);
|
|
|
|
/*
|
|
* Limit # of readers that can be woken up per wakeup call.
|
|
*/
|
|
if (woken >= MAX_READERS_WAKEUP)
|
|
break;
|
|
}
|
|
|
|
adjustment = woken * RWSEM_READER_BIAS - adjustment;
|
|
lockevent_cond_inc(rwsem_wake_reader, woken);
|
|
if (list_empty(&sem->wait_list)) {
|
|
/* hit end of list above */
|
|
adjustment -= RWSEM_FLAG_WAITERS;
|
|
}
|
|
|
|
/*
|
|
* When we've woken a reader, we no longer need to force writers
|
|
* to give up the lock and we can clear HANDOFF.
|
|
*/
|
|
if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
|
|
adjustment -= RWSEM_FLAG_HANDOFF;
|
|
|
|
if (adjustment)
|
|
atomic_long_add(adjustment, &sem->count);
|
|
|
|
/* 2nd pass */
|
|
list_for_each_entry_safe(waiter, tmp, &wlist, list) {
|
|
struct task_struct *tsk;
|
|
|
|
tsk = waiter->task;
|
|
get_task_struct(tsk);
|
|
|
|
/*
|
|
* Ensure calling get_task_struct() before setting the reader
|
|
* waiter to nil such that rwsem_down_read_slowpath() cannot
|
|
* race with do_exit() by always holding a reference count
|
|
* to the task to wakeup.
|
|
*/
|
|
smp_store_release(&waiter->task, NULL);
|
|
/*
|
|
* Ensure issuing the wakeup (either by us or someone else)
|
|
* after setting the reader waiter to nil.
|
|
*/
|
|
wake_q_add_safe(wake_q, tsk);
|
|
}
|
|
}
|
|
|
|
/*
|
|
* This function must be called with the sem->wait_lock held to prevent
|
|
* race conditions between checking the rwsem wait list and setting the
|
|
* sem->count accordingly.
|
|
*
|
|
* If wstate is WRITER_HANDOFF, it will make sure that either the handoff
|
|
* bit is set or the lock is acquired with handoff bit cleared.
|
|
*/
|
|
static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
|
|
enum writer_wait_state wstate)
|
|
{
|
|
long count, new;
|
|
|
|
lockdep_assert_held(&sem->wait_lock);
|
|
|
|
count = atomic_long_read(&sem->count);
|
|
do {
|
|
bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
|
|
|
|
if (has_handoff && wstate == WRITER_NOT_FIRST)
|
|
return false;
|
|
|
|
new = count;
|
|
|
|
if (count & RWSEM_LOCK_MASK) {
|
|
if (has_handoff || (wstate != WRITER_HANDOFF))
|
|
return false;
|
|
|
|
new |= RWSEM_FLAG_HANDOFF;
|
|
} else {
|
|
new |= RWSEM_WRITER_LOCKED;
|
|
new &= ~RWSEM_FLAG_HANDOFF;
|
|
|
|
if (list_is_singular(&sem->wait_list))
|
|
new &= ~RWSEM_FLAG_WAITERS;
|
|
}
|
|
} while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
|
|
|
|
/*
|
|
* We have either acquired the lock with handoff bit cleared or
|
|
* set the handoff bit.
|
|
*/
|
|
if (new & RWSEM_FLAG_HANDOFF)
|
|
return false;
|
|
|
|
rwsem_set_owner(sem);
|
|
return true;
|
|
}
|
|
|
|
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
|
|
/*
|
|
* Try to acquire read lock before the reader is put on wait queue.
|
|
* Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
|
|
* is ongoing.
|
|
*/
|
|
static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
|
|
{
|
|
long count = atomic_long_read(&sem->count);
|
|
|
|
if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))
|
|
return false;
|
|
|
|
count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
|
|
if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
|
|
rwsem_set_reader_owned(sem);
|
|
lockevent_inc(rwsem_opt_rlock);
|
|
return true;
|
|
}
|
|
|
|
/* Back out the change */
|
|
atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
|
|
return false;
|
|
}
|
|
|
|
/*
|
|
* Try to acquire write lock before the writer has been put on wait queue.
|
|
*/
|
|
static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
|
|
{
|
|
long count = atomic_long_read(&sem->count);
|
|
|
|
while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
|
|
if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
|
|
count | RWSEM_WRITER_LOCKED)) {
|
|
rwsem_set_owner(sem);
|
|
lockevent_inc(rwsem_opt_wlock);
|
|
return true;
|
|
}
|
|
}
|
|
return false;
|
|
}
|
|
|
|
static inline bool owner_on_cpu(struct task_struct *owner)
|
|
{
|
|
/*
|
|
* As lock holder preemption issue, we both skip spinning if
|
|
* task is not on cpu or its cpu is preempted
|
|
*/
|
|
return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
|
|
}
|
|
|
|
static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
|
|
unsigned long nonspinnable)
|
|
{
|
|
struct task_struct *owner;
|
|
unsigned long flags;
|
|
bool ret = true;
|
|
|
|
if (need_resched()) {
|
|
lockevent_inc(rwsem_opt_fail);
|
|
return false;
|
|
}
|
|
|
|
preempt_disable();
|
|
rcu_read_lock();
|
|
owner = rwsem_owner_flags(sem, &flags);
|
|
/*
|
|
* Don't check the read-owner as the entry may be stale.
|
|
*/
|
|
if ((flags & nonspinnable) ||
|
|
(owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
|
|
ret = false;
|
|
rcu_read_unlock();
|
|
preempt_enable();
|
|
|
|
lockevent_cond_inc(rwsem_opt_fail, !ret);
|
|
return ret;
|
|
}
|
|
|
|
/*
|
|
* The rwsem_spin_on_owner() function returns the folowing 4 values
|
|
* depending on the lock owner state.
|
|
* OWNER_NULL : owner is currently NULL
|
|
* OWNER_WRITER: when owner changes and is a writer
|
|
* OWNER_READER: when owner changes and the new owner may be a reader.
|
|
* OWNER_NONSPINNABLE:
|
|
* when optimistic spinning has to stop because either the
|
|
* owner stops running, is unknown, or its timeslice has
|
|
* been used up.
|
|
*/
|
|
enum owner_state {
|
|
OWNER_NULL = 1 << 0,
|
|
OWNER_WRITER = 1 << 1,
|
|
OWNER_READER = 1 << 2,
|
|
OWNER_NONSPINNABLE = 1 << 3,
|
|
};
|
|
#define OWNER_SPINNABLE (OWNER_NULL | OWNER_WRITER | OWNER_READER)
|
|
|
|
static inline enum owner_state
|
|
rwsem_owner_state(struct task_struct *owner, unsigned long flags, unsigned long nonspinnable)
|
|
{
|
|
if (flags & nonspinnable)
|
|
return OWNER_NONSPINNABLE;
|
|
|
|
if (flags & RWSEM_READER_OWNED)
|
|
return OWNER_READER;
|
|
|
|
return owner ? OWNER_WRITER : OWNER_NULL;
|
|
}
|
|
|
|
static noinline enum owner_state
|
|
rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
|
|
{
|
|
struct task_struct *new, *owner;
|
|
unsigned long flags, new_flags;
|
|
enum owner_state state;
|
|
|
|
owner = rwsem_owner_flags(sem, &flags);
|
|
state = rwsem_owner_state(owner, flags, nonspinnable);
|
|
if (state != OWNER_WRITER)
|
|
return state;
|
|
|
|
rcu_read_lock();
|
|
for (;;) {
|
|
/*
|
|
* When a waiting writer set the handoff flag, it may spin
|
|
* on the owner as well. Once that writer acquires the lock,
|
|
* we can spin on it. So we don't need to quit even when the
|
|
* handoff bit is set.
|
|
*/
|
|
new = rwsem_owner_flags(sem, &new_flags);
|
|
if ((new != owner) || (new_flags != flags)) {
|
|
state = rwsem_owner_state(new, new_flags, nonspinnable);
|
|
break;
|
|
}
|
|
|
|
/*
|
|
* Ensure we emit the owner->on_cpu, dereference _after_
|
|
* checking sem->owner still matches owner, if that fails,
|
|
* owner might point to free()d memory, if it still matches,
|
|
* the rcu_read_lock() ensures the memory stays valid.
|
|
*/
|
|
barrier();
|
|
|
|
if (need_resched() || !owner_on_cpu(owner)) {
|
|
state = OWNER_NONSPINNABLE;
|
|
break;
|
|
}
|
|
|
|
cpu_relax();
|
|
}
|
|
rcu_read_unlock();
|
|
|
|
return state;
|
|
}
|
|
|
|
/*
|
|
* Calculate reader-owned rwsem spinning threshold for writer
|
|
*
|
|
* The more readers own the rwsem, the longer it will take for them to
|
|
* wind down and free the rwsem. So the empirical formula used to
|
|
* determine the actual spinning time limit here is:
|
|
*
|
|
* Spinning threshold = (10 + nr_readers/2)us
|
|
*
|
|
* The limit is capped to a maximum of 25us (30 readers). This is just
|
|
* a heuristic and is subjected to change in the future.
|
|
*/
|
|
static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
|
|
{
|
|
long count = atomic_long_read(&sem->count);
|
|
int readers = count >> RWSEM_READER_SHIFT;
|
|
u64 delta;
|
|
|
|
if (readers > 30)
|
|
readers = 30;
|
|
delta = (20 + readers) * NSEC_PER_USEC / 2;
|
|
|
|
return sched_clock() + delta;
|
|
}
|
|
|
|
static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
|
|
{
|
|
bool taken = false;
|
|
int prev_owner_state = OWNER_NULL;
|
|
int loop = 0;
|
|
u64 rspin_threshold = 0;
|
|
unsigned long nonspinnable = wlock ? RWSEM_WR_NONSPINNABLE
|
|
: RWSEM_RD_NONSPINNABLE;
|
|
|
|
preempt_disable();
|
|
|
|
/* sem->wait_lock should not be held when doing optimistic spinning */
|
|
if (!osq_lock(&sem->osq))
|
|
goto done;
|
|
|
|
/*
|
|
* Optimistically spin on the owner field and attempt to acquire the
|
|
* lock whenever the owner changes. Spinning will be stopped when:
|
|
* 1) the owning writer isn't running; or
|
|
* 2) readers own the lock and spinning time has exceeded limit.
|
|
*/
|
|
for (;;) {
|
|
enum owner_state owner_state;
|
|
|
|
owner_state = rwsem_spin_on_owner(sem, nonspinnable);
|
|
if (!(owner_state & OWNER_SPINNABLE))
|
|
break;
|
|
|
|
/*
|
|
* Try to acquire the lock
|
|
*/
|
|
taken = wlock ? rwsem_try_write_lock_unqueued(sem)
|
|
: rwsem_try_read_lock_unqueued(sem);
|
|
|
|
if (taken)
|
|
break;
|
|
|
|
/*
|
|
* Time-based reader-owned rwsem optimistic spinning
|
|
*/
|
|
if (wlock && (owner_state == OWNER_READER)) {
|
|
/*
|
|
* Re-initialize rspin_threshold every time when
|
|
* the owner state changes from non-reader to reader.
|
|
* This allows a writer to steal the lock in between
|
|
* 2 reader phases and have the threshold reset at
|
|
* the beginning of the 2nd reader phase.
|
|
*/
|
|
if (prev_owner_state != OWNER_READER) {
|
|
if (rwsem_test_oflags(sem, nonspinnable))
|
|
break;
|
|
rspin_threshold = rwsem_rspin_threshold(sem);
|
|
loop = 0;
|
|
}
|
|
|
|
/*
|
|
* Check time threshold once every 16 iterations to
|
|
* avoid calling sched_clock() too frequently so
|
|
* as to reduce the average latency between the times
|
|
* when the lock becomes free and when the spinner
|
|
* is ready to do a trylock.
|
|
*/
|
|
else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) {
|
|
rwsem_set_nonspinnable(sem);
|
|
lockevent_inc(rwsem_opt_nospin);
|
|
break;
|
|
}
|
|
}
|
|
|
|
/*
|
|
* An RT task cannot do optimistic spinning if it cannot
|
|
* be sure the lock holder is running or live-lock may
|
|
* happen if the current task and the lock holder happen
|
|
* to run in the same CPU. However, aborting optimistic
|
|
* spinning while a NULL owner is detected may miss some
|
|
* opportunity where spinning can continue without causing
|
|
* problem.
|
|
*
|
|
* There are 2 possible cases where an RT task may be able
|
|
* to continue spinning.
|
|
*
|
|
* 1) The lock owner is in the process of releasing the
|
|
* lock, sem->owner is cleared but the lock has not
|
|
* been released yet.
|
|
* 2) The lock was free and owner cleared, but another
|
|
* task just comes in and acquire the lock before
|
|
* we try to get it. The new owner may be a spinnable
|
|
* writer.
|
|
*
|
|
* To take advantage of two scenarios listed agove, the RT
|
|
* task is made to retry one more time to see if it can
|
|
* acquire the lock or continue spinning on the new owning
|
|
* writer. Of course, if the time lag is long enough or the
|
|
* new owner is not a writer or spinnable, the RT task will
|
|
* quit spinning.
|
|
*
|
|
* If the owner is a writer, the need_resched() check is
|
|
* done inside rwsem_spin_on_owner(). If the owner is not
|
|
* a writer, need_resched() check needs to be done here.
|
|
*/
|
|
if (owner_state != OWNER_WRITER) {
|
|
if (need_resched())
|
|
break;
|
|
if (rt_task(current) &&
|
|
(prev_owner_state != OWNER_WRITER))
|
|
break;
|
|
}
|
|
prev_owner_state = owner_state;
|
|
|
|
/*
|
|
* The cpu_relax() call is a compiler barrier which forces
|
|
* everything in this loop to be re-loaded. We don't need
|
|
* memory barriers as we'll eventually observe the right
|
|
* values at the cost of a few extra spins.
|
|
*/
|
|
cpu_relax();
|
|
}
|
|
osq_unlock(&sem->osq);
|
|
done:
|
|
preempt_enable();
|
|
lockevent_cond_inc(rwsem_opt_fail, !taken);
|
|
return taken;
|
|
}
|
|
|
|
/*
|
|
* Clear the owner's RWSEM_WR_NONSPINNABLE bit if it is set. This should
|
|
* only be called when the reader count reaches 0.
|
|
*
|
|
* This give writers better chance to acquire the rwsem first before
|
|
* readers when the rwsem was being held by readers for a relatively long
|
|
* period of time. Race can happen that an optimistic spinner may have
|
|
* just stolen the rwsem and set the owner, but just clearing the
|
|
* RWSEM_WR_NONSPINNABLE bit will do no harm anyway.
|
|
*/
|
|
static inline void clear_wr_nonspinnable(struct rw_semaphore *sem)
|
|
{
|
|
if (rwsem_test_oflags(sem, RWSEM_WR_NONSPINNABLE))
|
|
atomic_long_andnot(RWSEM_WR_NONSPINNABLE, &sem->owner);
|
|
}
|
|
|
|
/*
|
|
* This function is called when the reader fails to acquire the lock via
|
|
* optimistic spinning. In this case we will still attempt to do a trylock
|
|
* when comparing the rwsem state right now with the state when entering
|
|
* the slowpath indicates that the reader is still in a valid reader phase.
|
|
* This happens when the following conditions are true:
|
|
*
|
|
* 1) The lock is currently reader owned, and
|
|
* 2) The lock is previously not reader-owned or the last read owner changes.
|
|
*
|
|
* In the former case, we have transitioned from a writer phase to a
|
|
* reader-phase while spinning. In the latter case, it means the reader
|
|
* phase hasn't ended when we entered the optimistic spinning loop. In
|
|
* both cases, the reader is eligible to acquire the lock. This is the
|
|
* secondary path where a read lock is acquired optimistically.
|
|
*
|
|
* The reader non-spinnable bit wasn't set at time of entry or it will
|
|
* not be here at all.
|
|
*/
|
|
static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
|
|
unsigned long last_rowner)
|
|
{
|
|
unsigned long owner = atomic_long_read(&sem->owner);
|
|
|
|
if (!(owner & RWSEM_READER_OWNED))
|
|
return false;
|
|
|
|
if (((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK) &&
|
|
rwsem_try_read_lock_unqueued(sem)) {
|
|
lockevent_inc(rwsem_opt_rlock2);
|
|
lockevent_add(rwsem_opt_fail, -1);
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
#else
|
|
static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
|
|
unsigned long nonspinnable)
|
|
{
|
|
return false;
|
|
}
|
|
|
|
static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
|
|
{
|
|
return false;
|
|
}
|
|
|
|
static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) { }
|
|
|
|
static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
|
|
unsigned long last_rowner)
|
|
{
|
|
return false;
|
|
}
|
|
|
|
static inline int
|
|
rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
|
|
{
|
|
return 0;
|
|
}
|
|
#define OWNER_NULL 1
|
|
#endif
|
|
|
|
/*
|
|
* Wait for the read lock to be granted
|
|
*/
|
|
static struct rw_semaphore __sched *
|
|
rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
|
|
{
|
|
long count, adjustment = -RWSEM_READER_BIAS;
|
|
struct rwsem_waiter waiter;
|
|
DEFINE_WAKE_Q(wake_q);
|
|
bool wake = false;
|
|
|
|
/*
|
|
* Save the current read-owner of rwsem, if available, and the
|
|
* reader nonspinnable bit.
|
|
*/
|
|
waiter.last_rowner = atomic_long_read(&sem->owner);
|
|
if (!(waiter.last_rowner & RWSEM_READER_OWNED))
|
|
waiter.last_rowner &= RWSEM_RD_NONSPINNABLE;
|
|
|
|
if (!rwsem_can_spin_on_owner(sem, RWSEM_RD_NONSPINNABLE))
|
|
goto queue;
|
|
|
|
/*
|
|
* Undo read bias from down_read() and do optimistic spinning.
|
|
*/
|
|
atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
|
|
adjustment = 0;
|
|
if (rwsem_optimistic_spin(sem, false)) {
|
|
/* rwsem_optimistic_spin() implies ACQUIRE on success */
|
|
/*
|
|
* Wake up other readers in the wait list if the front
|
|
* waiter is a reader.
|
|
*/
|
|
if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) {
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
if (!list_empty(&sem->wait_list))
|
|
rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
|
|
&wake_q);
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
wake_up_q(&wake_q);
|
|
}
|
|
return sem;
|
|
} else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
|
|
/* rwsem_reader_phase_trylock() implies ACQUIRE on success */
|
|
return sem;
|
|
}
|
|
|
|
queue:
|
|
waiter.task = current;
|
|
waiter.type = RWSEM_WAITING_FOR_READ;
|
|
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
if (list_empty(&sem->wait_list)) {
|
|
/*
|
|
* In case the wait queue is empty and the lock isn't owned
|
|
* by a writer or has the handoff bit set, this reader can
|
|
* exit the slowpath and return immediately as its
|
|
* RWSEM_READER_BIAS has already been set in the count.
|
|
*/
|
|
if (adjustment && !(atomic_long_read(&sem->count) &
|
|
(RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
|
|
/* Provide lock ACQUIRE */
|
|
smp_acquire__after_ctrl_dep();
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
rwsem_set_reader_owned(sem);
|
|
lockevent_inc(rwsem_rlock_fast);
|
|
return sem;
|
|
}
|
|
adjustment += RWSEM_FLAG_WAITERS;
|
|
}
|
|
list_add_tail(&waiter.list, &sem->wait_list);
|
|
|
|
/* we're now waiting on the lock, but no longer actively locking */
|
|
if (adjustment)
|
|
count = atomic_long_add_return(adjustment, &sem->count);
|
|
else
|
|
count = atomic_long_read(&sem->count);
|
|
|
|
/*
|
|
* If there are no active locks, wake the front queued process(es).
|
|
*
|
|
* If there are no writers and we are first in the queue,
|
|
* wake our own waiter to join the existing active readers !
|
|
*/
|
|
if (!(count & RWSEM_LOCK_MASK)) {
|
|
clear_wr_nonspinnable(sem);
|
|
wake = true;
|
|
}
|
|
if (wake || (!(count & RWSEM_WRITER_MASK) &&
|
|
(adjustment & RWSEM_FLAG_WAITERS)))
|
|
rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
|
|
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
wake_up_q(&wake_q);
|
|
|
|
/* wait to be given the lock */
|
|
for (;;) {
|
|
set_current_state(state);
|
|
if (!smp_load_acquire(&waiter.task)) {
|
|
/* Matches rwsem_mark_wake()'s smp_store_release(). */
|
|
break;
|
|
}
|
|
if (signal_pending_state(state, current)) {
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
if (waiter.task)
|
|
goto out_nolock;
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
/* Ordered by sem->wait_lock against rwsem_mark_wake(). */
|
|
break;
|
|
}
|
|
schedule();
|
|
lockevent_inc(rwsem_sleep_reader);
|
|
}
|
|
|
|
__set_current_state(TASK_RUNNING);
|
|
lockevent_inc(rwsem_rlock);
|
|
return sem;
|
|
|
|
out_nolock:
|
|
list_del(&waiter.list);
|
|
if (list_empty(&sem->wait_list)) {
|
|
atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
|
|
&sem->count);
|
|
}
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
__set_current_state(TASK_RUNNING);
|
|
lockevent_inc(rwsem_rlock_fail);
|
|
return ERR_PTR(-EINTR);
|
|
}
|
|
|
|
/*
|
|
* This function is called by the a write lock owner. So the owner value
|
|
* won't get changed by others.
|
|
*/
|
|
static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem,
|
|
bool disable)
|
|
{
|
|
if (unlikely(disable)) {
|
|
atomic_long_or(RWSEM_RD_NONSPINNABLE, &sem->owner);
|
|
lockevent_inc(rwsem_opt_norspin);
|
|
}
|
|
}
|
|
|
|
/*
|
|
* Wait until we successfully acquire the write lock
|
|
*/
|
|
static struct rw_semaphore *
|
|
rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
|
|
{
|
|
long count;
|
|
bool disable_rspin;
|
|
enum writer_wait_state wstate;
|
|
struct rwsem_waiter waiter;
|
|
struct rw_semaphore *ret = sem;
|
|
DEFINE_WAKE_Q(wake_q);
|
|
|
|
/* do optimistic spinning and steal lock if possible */
|
|
if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) &&
|
|
rwsem_optimistic_spin(sem, true)) {
|
|
/* rwsem_optimistic_spin() implies ACQUIRE on success */
|
|
return sem;
|
|
}
|
|
|
|
/*
|
|
* Disable reader optimistic spinning for this rwsem after
|
|
* acquiring the write lock when the setting of the nonspinnable
|
|
* bits are observed.
|
|
*/
|
|
disable_rspin = atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE;
|
|
|
|
/*
|
|
* Optimistic spinning failed, proceed to the slowpath
|
|
* and block until we can acquire the sem.
|
|
*/
|
|
waiter.task = current;
|
|
waiter.type = RWSEM_WAITING_FOR_WRITE;
|
|
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
|
|
/* account for this before adding a new element to the list */
|
|
wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;
|
|
|
|
list_add_tail(&waiter.list, &sem->wait_list);
|
|
|
|
/* we're now waiting on the lock */
|
|
if (wstate == WRITER_NOT_FIRST) {
|
|
count = atomic_long_read(&sem->count);
|
|
|
|
/*
|
|
* If there were already threads queued before us and:
|
|
* 1) there are no no active locks, wake the front
|
|
* queued process(es) as the handoff bit might be set.
|
|
* 2) there are no active writers and some readers, the lock
|
|
* must be read owned; so we try to wake any read lock
|
|
* waiters that were queued ahead of us.
|
|
*/
|
|
if (count & RWSEM_WRITER_MASK)
|
|
goto wait;
|
|
|
|
rwsem_mark_wake(sem, (count & RWSEM_READER_MASK)
|
|
? RWSEM_WAKE_READERS
|
|
: RWSEM_WAKE_ANY, &wake_q);
|
|
|
|
if (!wake_q_empty(&wake_q)) {
|
|
/*
|
|
* We want to minimize wait_lock hold time especially
|
|
* when a large number of readers are to be woken up.
|
|
*/
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
wake_up_q(&wake_q);
|
|
wake_q_init(&wake_q); /* Used again, reinit */
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
}
|
|
} else {
|
|
atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
|
|
}
|
|
|
|
wait:
|
|
/* wait until we successfully acquire the lock */
|
|
set_current_state(state);
|
|
for (;;) {
|
|
if (rwsem_try_write_lock(sem, wstate)) {
|
|
/* rwsem_try_write_lock() implies ACQUIRE on success */
|
|
break;
|
|
}
|
|
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
|
|
/*
|
|
* After setting the handoff bit and failing to acquire
|
|
* the lock, attempt to spin on owner to accelerate lock
|
|
* transfer. If the previous owner is a on-cpu writer and it
|
|
* has just released the lock, OWNER_NULL will be returned.
|
|
* In this case, we attempt to acquire the lock again
|
|
* without sleeping.
|
|
*/
|
|
if (wstate == WRITER_HANDOFF &&
|
|
rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)
|
|
goto trylock_again;
|
|
|
|
/* Block until there are no active lockers. */
|
|
for (;;) {
|
|
if (signal_pending_state(state, current))
|
|
goto out_nolock;
|
|
|
|
schedule();
|
|
lockevent_inc(rwsem_sleep_writer);
|
|
set_current_state(state);
|
|
/*
|
|
* If HANDOFF bit is set, unconditionally do
|
|
* a trylock.
|
|
*/
|
|
if (wstate == WRITER_HANDOFF)
|
|
break;
|
|
|
|
if ((wstate == WRITER_NOT_FIRST) &&
|
|
(rwsem_first_waiter(sem) == &waiter))
|
|
wstate = WRITER_FIRST;
|
|
|
|
count = atomic_long_read(&sem->count);
|
|
if (!(count & RWSEM_LOCK_MASK))
|
|
break;
|
|
|
|
/*
|
|
* The setting of the handoff bit is deferred
|
|
* until rwsem_try_write_lock() is called.
|
|
*/
|
|
if ((wstate == WRITER_FIRST) && (rt_task(current) ||
|
|
time_after(jiffies, waiter.timeout))) {
|
|
wstate = WRITER_HANDOFF;
|
|
lockevent_inc(rwsem_wlock_handoff);
|
|
break;
|
|
}
|
|
}
|
|
trylock_again:
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
}
|
|
__set_current_state(TASK_RUNNING);
|
|
list_del(&waiter.list);
|
|
rwsem_disable_reader_optspin(sem, disable_rspin);
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
lockevent_inc(rwsem_wlock);
|
|
|
|
return ret;
|
|
|
|
out_nolock:
|
|
__set_current_state(TASK_RUNNING);
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
list_del(&waiter.list);
|
|
|
|
if (unlikely(wstate == WRITER_HANDOFF))
|
|
atomic_long_add(-RWSEM_FLAG_HANDOFF, &sem->count);
|
|
|
|
if (list_empty(&sem->wait_list))
|
|
atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
|
|
else
|
|
rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
wake_up_q(&wake_q);
|
|
lockevent_inc(rwsem_wlock_fail);
|
|
|
|
return ERR_PTR(-EINTR);
|
|
}
|
|
|
|
/*
|
|
* handle waking up a waiter on the semaphore
|
|
* - up_read/up_write has decremented the active part of count if we come here
|
|
*/
|
|
static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count)
|
|
{
|
|
unsigned long flags;
|
|
DEFINE_WAKE_Q(wake_q);
|
|
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
|
|
if (!list_empty(&sem->wait_list))
|
|
rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
|
|
|
|
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
|
|
wake_up_q(&wake_q);
|
|
|
|
return sem;
|
|
}
|
|
|
|
/*
|
|
* downgrade a write lock into a read lock
|
|
* - caller incremented waiting part of count and discovered it still negative
|
|
* - just wake up any readers at the front of the queue
|
|
*/
|
|
static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
|
|
{
|
|
unsigned long flags;
|
|
DEFINE_WAKE_Q(wake_q);
|
|
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
|
|
if (!list_empty(&sem->wait_list))
|
|
rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
|
|
|
|
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
|
|
wake_up_q(&wake_q);
|
|
|
|
return sem;
|
|
}
|
|
|
|
/*
|
|
* lock for reading
|
|
*/
|
|
static inline void __down_read(struct rw_semaphore *sem)
|
|
{
|
|
if (!rwsem_read_trylock(sem)) {
|
|
rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
|
|
DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
|
|
} else {
|
|
rwsem_set_reader_owned(sem);
|
|
}
|
|
}
|
|
|
|
static inline int __down_read_killable(struct rw_semaphore *sem)
|
|
{
|
|
if (!rwsem_read_trylock(sem)) {
|
|
if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))
|
|
return -EINTR;
|
|
DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
|
|
} else {
|
|
rwsem_set_reader_owned(sem);
|
|
}
|
|
return 0;
|
|
}
|
|
|
|
static inline int __down_read_trylock(struct rw_semaphore *sem)
|
|
{
|
|
long tmp;
|
|
|
|
DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
|
|
|
|
/*
|
|
* Optimize for the case when the rwsem is not locked at all.
|
|
*/
|
|
tmp = RWSEM_UNLOCKED_VALUE;
|
|
do {
|
|
if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
|
|
tmp + RWSEM_READER_BIAS)) {
|
|
rwsem_set_reader_owned(sem);
|
|
return 1;
|
|
}
|
|
} while (!(tmp & RWSEM_READ_FAILED_MASK));
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* lock for writing
|
|
*/
|
|
static inline void __down_write(struct rw_semaphore *sem)
|
|
{
|
|
long tmp = RWSEM_UNLOCKED_VALUE;
|
|
|
|
if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
|
|
RWSEM_WRITER_LOCKED)))
|
|
rwsem_down_write_slowpath(sem, TASK_UNINTERRUPTIBLE);
|
|
else
|
|
rwsem_set_owner(sem);
|
|
}
|
|
|
|
static inline int __down_write_killable(struct rw_semaphore *sem)
|
|
{
|
|
long tmp = RWSEM_UNLOCKED_VALUE;
|
|
|
|
if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
|
|
RWSEM_WRITER_LOCKED))) {
|
|
if (IS_ERR(rwsem_down_write_slowpath(sem, TASK_KILLABLE)))
|
|
return -EINTR;
|
|
} else {
|
|
rwsem_set_owner(sem);
|
|
}
|
|
return 0;
|
|
}
|
|
|
|
static inline int __down_write_trylock(struct rw_semaphore *sem)
|
|
{
|
|
long tmp;
|
|
|
|
DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
|
|
|
|
tmp = RWSEM_UNLOCKED_VALUE;
|
|
if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
|
|
RWSEM_WRITER_LOCKED)) {
|
|
rwsem_set_owner(sem);
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
/*
|
|
* unlock after reading
|
|
*/
|
|
static inline void __up_read(struct rw_semaphore *sem)
|
|
{
|
|
long tmp;
|
|
|
|
DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
|
|
DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
|
|
|
|
rwsem_clear_reader_owned(sem);
|
|
tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
|
|
DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
|
|
if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
|
|
RWSEM_FLAG_WAITERS)) {
|
|
clear_wr_nonspinnable(sem);
|
|
rwsem_wake(sem, tmp);
|
|
}
|
|
}
|
|
|
|
/*
|
|
* unlock after writing
|
|
*/
|
|
static inline void __up_write(struct rw_semaphore *sem)
|
|
{
|
|
long tmp;
|
|
|
|
DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
|
|
/*
|
|
* sem->owner may differ from current if the ownership is transferred
|
|
* to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
|
|
*/
|
|
DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
|
|
!rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);
|
|
|
|
rwsem_clear_owner(sem);
|
|
tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
|
|
if (unlikely(tmp & RWSEM_FLAG_WAITERS))
|
|
rwsem_wake(sem, tmp);
|
|
}
|
|
|
|
/*
|
|
* downgrade write lock to read lock
|
|
*/
|
|
static inline void __downgrade_write(struct rw_semaphore *sem)
|
|
{
|
|
long tmp;
|
|
|
|
/*
|
|
* When downgrading from exclusive to shared ownership,
|
|
* anything inside the write-locked region cannot leak
|
|
* into the read side. In contrast, anything in the
|
|
* read-locked region is ok to be re-ordered into the
|
|
* write side. As such, rely on RELEASE semantics.
|
|
*/
|
|
DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);
|
|
tmp = atomic_long_fetch_add_release(
|
|
-RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
|
|
rwsem_set_reader_owned(sem);
|
|
if (tmp & RWSEM_FLAG_WAITERS)
|
|
rwsem_downgrade_wake(sem);
|
|
}
|
|
|
|
/*
|
|
* lock for reading
|
|
*/
|
|
void __sched down_read(struct rw_semaphore *sem)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
|
|
|
|
LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
|
|
}
|
|
EXPORT_SYMBOL(down_read);
|
|
|
|
int __sched down_read_killable(struct rw_semaphore *sem)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
|
|
|
|
if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
|
|
rwsem_release(&sem->dep_map, _RET_IP_);
|
|
return -EINTR;
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
EXPORT_SYMBOL(down_read_killable);
|
|
|
|
/*
|
|
* trylock for reading -- returns 1 if successful, 0 if contention
|
|
*/
|
|
int down_read_trylock(struct rw_semaphore *sem)
|
|
{
|
|
int ret = __down_read_trylock(sem);
|
|
|
|
if (ret == 1)
|
|
rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
|
|
return ret;
|
|
}
|
|
EXPORT_SYMBOL(down_read_trylock);
|
|
|
|
/*
|
|
* lock for writing
|
|
*/
|
|
void __sched down_write(struct rw_semaphore *sem)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
|
|
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
|
|
}
|
|
EXPORT_SYMBOL(down_write);
|
|
|
|
/*
|
|
* lock for writing
|
|
*/
|
|
int __sched down_write_killable(struct rw_semaphore *sem)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
|
|
|
|
if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
|
|
__down_write_killable)) {
|
|
rwsem_release(&sem->dep_map, _RET_IP_);
|
|
return -EINTR;
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
EXPORT_SYMBOL(down_write_killable);
|
|
|
|
/*
|
|
* trylock for writing -- returns 1 if successful, 0 if contention
|
|
*/
|
|
int down_write_trylock(struct rw_semaphore *sem)
|
|
{
|
|
int ret = __down_write_trylock(sem);
|
|
|
|
if (ret == 1)
|
|
rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
|
|
|
|
return ret;
|
|
}
|
|
EXPORT_SYMBOL(down_write_trylock);
|
|
|
|
/*
|
|
* release a read lock
|
|
*/
|
|
void up_read(struct rw_semaphore *sem)
|
|
{
|
|
rwsem_release(&sem->dep_map, _RET_IP_);
|
|
__up_read(sem);
|
|
}
|
|
EXPORT_SYMBOL(up_read);
|
|
|
|
/*
|
|
* release a write lock
|
|
*/
|
|
void up_write(struct rw_semaphore *sem)
|
|
{
|
|
rwsem_release(&sem->dep_map, _RET_IP_);
|
|
__up_write(sem);
|
|
}
|
|
EXPORT_SYMBOL(up_write);
|
|
|
|
/*
|
|
* downgrade write lock to read lock
|
|
*/
|
|
void downgrade_write(struct rw_semaphore *sem)
|
|
{
|
|
lock_downgrade(&sem->dep_map, _RET_IP_);
|
|
__downgrade_write(sem);
|
|
}
|
|
EXPORT_SYMBOL(downgrade_write);
|
|
|
|
#ifdef CONFIG_DEBUG_LOCK_ALLOC
|
|
|
|
void down_read_nested(struct rw_semaphore *sem, int subclass)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
|
|
LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
|
|
}
|
|
EXPORT_SYMBOL(down_read_nested);
|
|
|
|
void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
|
|
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
|
|
}
|
|
EXPORT_SYMBOL(_down_write_nest_lock);
|
|
|
|
void down_read_non_owner(struct rw_semaphore *sem)
|
|
{
|
|
might_sleep();
|
|
__down_read(sem);
|
|
__rwsem_set_reader_owned(sem, NULL);
|
|
}
|
|
EXPORT_SYMBOL(down_read_non_owner);
|
|
|
|
void down_write_nested(struct rw_semaphore *sem, int subclass)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
|
|
LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
|
|
}
|
|
EXPORT_SYMBOL(down_write_nested);
|
|
|
|
int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
|
|
{
|
|
might_sleep();
|
|
rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
|
|
|
|
if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
|
|
__down_write_killable)) {
|
|
rwsem_release(&sem->dep_map, _RET_IP_);
|
|
return -EINTR;
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
EXPORT_SYMBOL(down_write_killable_nested);
|
|
|
|
void up_read_non_owner(struct rw_semaphore *sem)
|
|
{
|
|
DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
|
|
__up_read(sem);
|
|
}
|
|
EXPORT_SYMBOL(up_read_non_owner);
|
|
|
|
#endif
|