![Jason Low](/assets/img/avatar_default.png)
Commit9b0fc9c09f
("rwsem: skip initial trylock in rwsem_down_write_failed") checks for if there are known active lockers in order to avoid write trylocking using expensive cmpxchg() when it likely wouldn't get the lock. However, a subsequent patch was added such that we directly check for sem->count == RWSEM_WAITING_BIAS right before trying that cmpxchg(). Thus, commit9b0fc9c09f
now just adds overhead. This patch modifies it so that we only do a check for if count == RWSEM_WAITING_BIAS. Also, add a comment on why we do an "extra check" of count before the cmpxchg(). Signed-off-by: Jason Low <jason.low2@hp.com> Acked-by: Davidlohr Bueso <dbueso@suse.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Chegu Vinod <chegu_vinod@hp.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/1410913017.2447.22.camel@j-VirtualBox Signed-off-by: Ingo Molnar <mingo@kernel.org>
515 lines
14 KiB
C
515 lines
14 KiB
C
/* rwsem.c: R/W semaphores: contention handling functions
|
|
*
|
|
* Written by David Howells (dhowells@redhat.com).
|
|
* Derived from arch/i386/kernel/semaphore.c
|
|
*
|
|
* 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.
|
|
*/
|
|
#include <linux/rwsem.h>
|
|
#include <linux/sched.h>
|
|
#include <linux/init.h>
|
|
#include <linux/export.h>
|
|
#include <linux/sched/rt.h>
|
|
|
|
#include "mcs_spinlock.h"
|
|
|
|
/*
|
|
* Guide to the rw_semaphore's count field for common values.
|
|
* (32-bit case illustrated, similar for 64-bit)
|
|
*
|
|
* 0x0000000X (1) X readers active or attempting lock, no writer waiting
|
|
* X = #active_readers + #readers attempting to lock
|
|
* (X*ACTIVE_BIAS)
|
|
*
|
|
* 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
|
|
* attempting to read lock or write lock.
|
|
*
|
|
* 0xffff000X (1) X readers active or attempting lock, with waiters for lock
|
|
* X = #active readers + # readers attempting lock
|
|
* (X*ACTIVE_BIAS + WAITING_BIAS)
|
|
* (2) 1 writer attempting lock, no waiters for lock
|
|
* X-1 = #active readers + #readers attempting lock
|
|
* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
|
|
* (3) 1 writer active, no waiters for lock
|
|
* X-1 = #active readers + #readers attempting lock
|
|
* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
|
|
*
|
|
* 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
|
|
* (WAITING_BIAS + ACTIVE_BIAS)
|
|
* (2) 1 writer active or attempting lock, no waiters for lock
|
|
* (ACTIVE_WRITE_BIAS)
|
|
*
|
|
* 0xffff0000 (1) There are writers or readers queued but none active
|
|
* or in the process of attempting lock.
|
|
* (WAITING_BIAS)
|
|
* Note: writer can attempt to steal lock for this count by adding
|
|
* ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
|
|
*
|
|
* 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
|
|
* (ACTIVE_WRITE_BIAS + WAITING_BIAS)
|
|
*
|
|
* Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
|
|
* the count becomes more than 0 for successful lock acquisition,
|
|
* i.e. the case where there are only readers or nobody has lock.
|
|
* (1st and 2nd case above).
|
|
*
|
|
* Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
|
|
* checking the count becomes ACTIVE_WRITE_BIAS for successful lock
|
|
* acquisition (i.e. nobody else has lock or attempts lock). If
|
|
* unsuccessful, in rwsem_down_write_failed, we'll check to see if there
|
|
* are only waiters but none active (5th case above), and attempt to
|
|
* steal the lock.
|
|
*
|
|
*/
|
|
|
|
/*
|
|
* 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(&sem->dep_map, name, key, 0);
|
|
#endif
|
|
sem->count = RWSEM_UNLOCKED_VALUE;
|
|
raw_spin_lock_init(&sem->wait_lock);
|
|
INIT_LIST_HEAD(&sem->wait_list);
|
|
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
|
|
sem->owner = NULL;
|
|
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;
|
|
};
|
|
|
|
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 */
|
|
};
|
|
|
|
/*
|
|
* handle the lock release when processes blocked on it that can now run
|
|
* - if we come here from up_xxxx(), then:
|
|
* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
|
|
* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
|
|
* - there must be someone on the queue
|
|
* - the spinlock must be held by the caller
|
|
* - woken process blocks are discarded from the list after having task zeroed
|
|
* - writers are only woken if downgrading is false
|
|
*/
|
|
static struct rw_semaphore *
|
|
__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
|
|
{
|
|
struct rwsem_waiter *waiter;
|
|
struct task_struct *tsk;
|
|
struct list_head *next;
|
|
long oldcount, woken, loop, adjustment;
|
|
|
|
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
|
|
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
|
|
if (wake_type == RWSEM_WAKE_ANY)
|
|
/* Wake writer at the front of the queue, but do not
|
|
* grant it the lock yet as we want other writers
|
|
* to be able to steal it. Readers, on the other hand,
|
|
* will block as they will notice the queued writer.
|
|
*/
|
|
wake_up_process(waiter->task);
|
|
goto out;
|
|
}
|
|
|
|
/* 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.
|
|
*/
|
|
adjustment = 0;
|
|
if (wake_type != RWSEM_WAKE_READ_OWNED) {
|
|
adjustment = RWSEM_ACTIVE_READ_BIAS;
|
|
try_reader_grant:
|
|
oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
|
|
if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
|
|
/* A writer stole the lock. Undo our reader grant. */
|
|
if (rwsem_atomic_update(-adjustment, sem) &
|
|
RWSEM_ACTIVE_MASK)
|
|
goto out;
|
|
/* Last active locker left. Retry waking readers. */
|
|
goto try_reader_grant;
|
|
}
|
|
}
|
|
|
|
/* Grant an infinite number of read locks to the readers at the front
|
|
* of the queue. Note we increment the 'active part' of the count by
|
|
* the number of readers before waking any processes up.
|
|
*/
|
|
woken = 0;
|
|
do {
|
|
woken++;
|
|
|
|
if (waiter->list.next == &sem->wait_list)
|
|
break;
|
|
|
|
waiter = list_entry(waiter->list.next,
|
|
struct rwsem_waiter, list);
|
|
|
|
} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
|
|
|
|
adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
|
|
if (waiter->type != RWSEM_WAITING_FOR_WRITE)
|
|
/* hit end of list above */
|
|
adjustment -= RWSEM_WAITING_BIAS;
|
|
|
|
if (adjustment)
|
|
rwsem_atomic_add(adjustment, sem);
|
|
|
|
next = sem->wait_list.next;
|
|
loop = woken;
|
|
do {
|
|
waiter = list_entry(next, struct rwsem_waiter, list);
|
|
next = waiter->list.next;
|
|
tsk = waiter->task;
|
|
smp_mb();
|
|
waiter->task = NULL;
|
|
wake_up_process(tsk);
|
|
put_task_struct(tsk);
|
|
} while (--loop);
|
|
|
|
sem->wait_list.next = next;
|
|
next->prev = &sem->wait_list;
|
|
|
|
out:
|
|
return sem;
|
|
}
|
|
|
|
/*
|
|
* Wait for the read lock to be granted
|
|
*/
|
|
__visible
|
|
struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
|
|
{
|
|
long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
|
|
struct rwsem_waiter waiter;
|
|
struct task_struct *tsk = current;
|
|
|
|
/* set up my own style of waitqueue */
|
|
waiter.task = tsk;
|
|
waiter.type = RWSEM_WAITING_FOR_READ;
|
|
get_task_struct(tsk);
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
if (list_empty(&sem->wait_list))
|
|
adjustment += RWSEM_WAITING_BIAS;
|
|
list_add_tail(&waiter.list, &sem->wait_list);
|
|
|
|
/* we're now waiting on the lock, but no longer actively locking */
|
|
count = rwsem_atomic_update(adjustment, sem);
|
|
|
|
/* 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_WAITING_BIAS ||
|
|
(count > RWSEM_WAITING_BIAS &&
|
|
adjustment != -RWSEM_ACTIVE_READ_BIAS))
|
|
sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
|
|
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
|
|
/* wait to be given the lock */
|
|
while (true) {
|
|
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
|
|
if (!waiter.task)
|
|
break;
|
|
schedule();
|
|
}
|
|
|
|
tsk->state = TASK_RUNNING;
|
|
|
|
return sem;
|
|
}
|
|
EXPORT_SYMBOL(rwsem_down_read_failed);
|
|
|
|
static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
|
|
{
|
|
/*
|
|
* Try acquiring the write lock. Check count first in order
|
|
* to reduce unnecessary expensive cmpxchg() operations.
|
|
*/
|
|
if (count == RWSEM_WAITING_BIAS &&
|
|
cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
|
|
RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
|
|
if (!list_is_singular(&sem->wait_list))
|
|
rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
|
|
return true;
|
|
}
|
|
|
|
return false;
|
|
}
|
|
|
|
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
|
|
/*
|
|
* 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 old, count = ACCESS_ONCE(sem->count);
|
|
|
|
while (true) {
|
|
if (!(count == 0 || count == RWSEM_WAITING_BIAS))
|
|
return false;
|
|
|
|
old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS);
|
|
if (old == count)
|
|
return true;
|
|
|
|
count = old;
|
|
}
|
|
}
|
|
|
|
static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
|
|
{
|
|
struct task_struct *owner;
|
|
bool on_cpu = false;
|
|
|
|
if (need_resched())
|
|
return false;
|
|
|
|
rcu_read_lock();
|
|
owner = ACCESS_ONCE(sem->owner);
|
|
if (owner)
|
|
on_cpu = owner->on_cpu;
|
|
rcu_read_unlock();
|
|
|
|
/*
|
|
* If sem->owner is not set, yet we have just recently entered the
|
|
* slowpath, then there is a possibility reader(s) may have the lock.
|
|
* To be safe, avoid spinning in these situations.
|
|
*/
|
|
return on_cpu;
|
|
}
|
|
|
|
static inline bool owner_running(struct rw_semaphore *sem,
|
|
struct task_struct *owner)
|
|
{
|
|
if (sem->owner != owner)
|
|
return false;
|
|
|
|
/*
|
|
* 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();
|
|
|
|
return owner->on_cpu;
|
|
}
|
|
|
|
static noinline
|
|
bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
|
|
{
|
|
rcu_read_lock();
|
|
while (owner_running(sem, owner)) {
|
|
if (need_resched())
|
|
break;
|
|
|
|
cpu_relax_lowlatency();
|
|
}
|
|
rcu_read_unlock();
|
|
|
|
/*
|
|
* We break out the loop above on need_resched() or when the
|
|
* owner changed, which is a sign for heavy contention. Return
|
|
* success only when sem->owner is NULL.
|
|
*/
|
|
return sem->owner == NULL;
|
|
}
|
|
|
|
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
|
|
{
|
|
struct task_struct *owner;
|
|
bool taken = false;
|
|
|
|
preempt_disable();
|
|
|
|
/* sem->wait_lock should not be held when doing optimistic spinning */
|
|
if (!rwsem_can_spin_on_owner(sem))
|
|
goto done;
|
|
|
|
if (!osq_lock(&sem->osq))
|
|
goto done;
|
|
|
|
while (true) {
|
|
owner = ACCESS_ONCE(sem->owner);
|
|
if (owner && !rwsem_spin_on_owner(sem, owner))
|
|
break;
|
|
|
|
/* wait_lock will be acquired if write_lock is obtained */
|
|
if (rwsem_try_write_lock_unqueued(sem)) {
|
|
taken = true;
|
|
break;
|
|
}
|
|
|
|
/*
|
|
* When there's no owner, we might have preempted between the
|
|
* owner acquiring the lock and setting the owner field. If
|
|
* we're an RT task that will live-lock because we won't let
|
|
* the owner complete.
|
|
*/
|
|
if (!owner && (need_resched() || rt_task(current)))
|
|
break;
|
|
|
|
/*
|
|
* 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_lowlatency();
|
|
}
|
|
osq_unlock(&sem->osq);
|
|
done:
|
|
preempt_enable();
|
|
return taken;
|
|
}
|
|
|
|
#else
|
|
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
|
|
{
|
|
return false;
|
|
}
|
|
#endif
|
|
|
|
/*
|
|
* Wait until we successfully acquire the write lock
|
|
*/
|
|
__visible
|
|
struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
|
|
{
|
|
long count;
|
|
bool waiting = true; /* any queued threads before us */
|
|
struct rwsem_waiter waiter;
|
|
|
|
/* undo write bias from down_write operation, stop active locking */
|
|
count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem);
|
|
|
|
/* do optimistic spinning and steal lock if possible */
|
|
if (rwsem_optimistic_spin(sem))
|
|
return sem;
|
|
|
|
/*
|
|
* Optimistic spinning failed, proceed to the slowpath
|
|
* and block until we can acquire the sem.
|
|
*/
|
|
waiter.task = current;
|
|
waiter.type = RWSEM_WAITING_FOR_WRITE;
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
|
|
/* account for this before adding a new element to the list */
|
|
if (list_empty(&sem->wait_list))
|
|
waiting = false;
|
|
|
|
list_add_tail(&waiter.list, &sem->wait_list);
|
|
|
|
/* we're now waiting on the lock, but no longer actively locking */
|
|
if (waiting) {
|
|
count = ACCESS_ONCE(sem->count);
|
|
|
|
/*
|
|
* If there were already threads queued before us and there are
|
|
* no active writers, the lock must be read owned; so we try to
|
|
* wake any read locks that were queued ahead of us.
|
|
*/
|
|
if (count > RWSEM_WAITING_BIAS)
|
|
sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
|
|
|
|
} else
|
|
count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
|
|
|
|
/* wait until we successfully acquire the lock */
|
|
set_current_state(TASK_UNINTERRUPTIBLE);
|
|
while (true) {
|
|
if (rwsem_try_write_lock(count, sem))
|
|
break;
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
|
|
/* Block until there are no active lockers. */
|
|
do {
|
|
schedule();
|
|
set_current_state(TASK_UNINTERRUPTIBLE);
|
|
} while ((count = sem->count) & RWSEM_ACTIVE_MASK);
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
}
|
|
__set_current_state(TASK_RUNNING);
|
|
|
|
list_del(&waiter.list);
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
|
|
return sem;
|
|
}
|
|
EXPORT_SYMBOL(rwsem_down_write_failed);
|
|
|
|
/*
|
|
* handle waking up a waiter on the semaphore
|
|
* - up_read/up_write has decremented the active part of count if we come here
|
|
*/
|
|
__visible
|
|
struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
|
|
{
|
|
unsigned long flags;
|
|
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
|
|
/* do nothing if list empty */
|
|
if (!list_empty(&sem->wait_list))
|
|
sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
|
|
|
|
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
|
|
|
|
return sem;
|
|
}
|
|
EXPORT_SYMBOL(rwsem_wake);
|
|
|
|
/*
|
|
* 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
|
|
*/
|
|
__visible
|
|
struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
|
|
{
|
|
unsigned long flags;
|
|
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
|
|
/* do nothing if list empty */
|
|
if (!list_empty(&sem->wait_list))
|
|
sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
|
|
|
|
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
|
|
|
|
return sem;
|
|
}
|
|
EXPORT_SYMBOL(rwsem_downgrade_wake);
|