![Waiman Long](/assets/img/avatar_default.png)
During my rwsem testing, it was found that after a down_read(), the reader count may occasionally become 0 or even negative. Consequently, a writer may steal the lock at that time and execute with the reader in parallel thus breaking the mutual exclusion guarantee of the write lock. In other words, both readers and writer can become rwsem owners simultaneously. The current reader wakeup code does it in one pass to clear waiter->task and put them into wake_q before fully incrementing the reader count. Once waiter->task is cleared, the corresponding reader may see it, finish the critical section and do unlock to decrement the count before the count is incremented. This is not a problem if there is only one reader to wake up as the count has been pre-incremented by 1. It is a problem if there are more than one readers to be woken up and writer can steal the lock. The wakeup was actually done in 2 passes before the following v4.9 commit:70800c3c0c
("locking/rwsem: Scan the wait_list for readers only once") To fix this problem, the wakeup is now done in two passes again. In the first pass, we collect the readers and count them. The reader count is then fully incremented. In the second pass, the waiter->task is then cleared and they are put into wake_q to be woken up later. Signed-off-by: Waiman Long <longman@redhat.com> Acked-by: Linus Torvalds <torvalds@linux-foundation.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Will Deacon <will.deacon@arm.com> Cc: huang ying <huang.ying.caritas@gmail.com> Fixes:70800c3c0c
("locking/rwsem: Scan the wait_list for readers only once") Link: http://lkml.kernel.org/r/20190428212557.13482-2-longman@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
746 lines
21 KiB
C
746 lines
21 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/* rwsem.c: R/W semaphores: contention handling functions
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*
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* Written by David Howells (dhowells@redhat.com).
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* Derived from arch/i386/kernel/semaphore.c
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*
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* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
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* and Michel Lespinasse <walken@google.com>
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*
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* Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
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* and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
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*/
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#include <linux/rwsem.h>
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#include <linux/init.h>
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#include <linux/export.h>
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#include <linux/sched/signal.h>
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#include <linux/sched/rt.h>
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#include <linux/sched/wake_q.h>
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#include <linux/sched/debug.h>
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#include <linux/osq_lock.h>
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#include "rwsem.h"
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/*
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* Guide to the rw_semaphore's count field for common values.
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* (32-bit case illustrated, similar for 64-bit)
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*
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* 0x0000000X (1) X readers active or attempting lock, no writer waiting
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* X = #active_readers + #readers attempting to lock
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* (X*ACTIVE_BIAS)
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*
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* 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
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* attempting to read lock or write lock.
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*
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* 0xffff000X (1) X readers active or attempting lock, with waiters for lock
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* X = #active readers + # readers attempting lock
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* (X*ACTIVE_BIAS + WAITING_BIAS)
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* (2) 1 writer attempting lock, no waiters for lock
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* X-1 = #active readers + #readers attempting lock
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* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
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* (3) 1 writer active, no waiters for lock
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* X-1 = #active readers + #readers attempting lock
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* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
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*
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* 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
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* (WAITING_BIAS + ACTIVE_BIAS)
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* (2) 1 writer active or attempting lock, no waiters for lock
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* (ACTIVE_WRITE_BIAS)
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*
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* 0xffff0000 (1) There are writers or readers queued but none active
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* or in the process of attempting lock.
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* (WAITING_BIAS)
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* Note: writer can attempt to steal lock for this count by adding
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* ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
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*
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* 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
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* (ACTIVE_WRITE_BIAS + WAITING_BIAS)
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*
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* Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
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* the count becomes more than 0 for successful lock acquisition,
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* i.e. the case where there are only readers or nobody has lock.
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* (1st and 2nd case above).
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*
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* Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
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* checking the count becomes ACTIVE_WRITE_BIAS for successful lock
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* acquisition (i.e. nobody else has lock or attempts lock). If
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* unsuccessful, in rwsem_down_write_failed, we'll check to see if there
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* are only waiters but none active (5th case above), and attempt to
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* steal the lock.
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*
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*/
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/*
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* Initialize an rwsem:
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*/
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void __init_rwsem(struct rw_semaphore *sem, const char *name,
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struct lock_class_key *key)
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{
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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/*
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* Make sure we are not reinitializing a held semaphore:
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*/
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debug_check_no_locks_freed((void *)sem, sizeof(*sem));
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lockdep_init_map(&sem->dep_map, name, key, 0);
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#endif
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atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
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raw_spin_lock_init(&sem->wait_lock);
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INIT_LIST_HEAD(&sem->wait_list);
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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sem->owner = NULL;
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osq_lock_init(&sem->osq);
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#endif
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}
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EXPORT_SYMBOL(__init_rwsem);
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enum rwsem_waiter_type {
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RWSEM_WAITING_FOR_WRITE,
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RWSEM_WAITING_FOR_READ
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};
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struct rwsem_waiter {
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struct list_head list;
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struct task_struct *task;
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enum rwsem_waiter_type type;
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};
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enum rwsem_wake_type {
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RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
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RWSEM_WAKE_READERS, /* Wake readers only */
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RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
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};
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/*
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* handle the lock release when processes blocked on it that can now run
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* - if we come here from up_xxxx(), then:
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* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
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* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
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* - there must be someone on the queue
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* - the wait_lock must be held by the caller
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* - tasks are marked for wakeup, the caller must later invoke wake_up_q()
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* to actually wakeup the blocked task(s) and drop the reference count,
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* preferably when the wait_lock is released
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* - woken process blocks are discarded from the list after having task zeroed
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* - writers are only marked woken if downgrading is false
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*/
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static void __rwsem_mark_wake(struct rw_semaphore *sem,
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enum rwsem_wake_type wake_type,
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struct wake_q_head *wake_q)
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{
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struct rwsem_waiter *waiter, *tmp;
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long oldcount, woken = 0, adjustment = 0;
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struct list_head wlist;
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/*
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* Take a peek at the queue head waiter such that we can determine
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* the wakeup(s) to perform.
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*/
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waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list);
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if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
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if (wake_type == RWSEM_WAKE_ANY) {
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/*
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* Mark writer at the front of the queue for wakeup.
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* Until the task is actually later awoken later by
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* the caller, other writers are able to steal it.
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* Readers, on the other hand, will block as they
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* will notice the queued writer.
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*/
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wake_q_add(wake_q, waiter->task);
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lockevent_inc(rwsem_wake_writer);
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}
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return;
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}
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/*
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* Writers might steal the lock before we grant it to the next reader.
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* We prefer to do the first reader grant before counting readers
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* so we can bail out early if a writer stole the lock.
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*/
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if (wake_type != RWSEM_WAKE_READ_OWNED) {
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adjustment = RWSEM_ACTIVE_READ_BIAS;
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try_reader_grant:
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oldcount = atomic_long_fetch_add(adjustment, &sem->count);
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if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
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/*
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* If the count is still less than RWSEM_WAITING_BIAS
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* after removing the adjustment, it is assumed that
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* a writer has stolen the lock. We have to undo our
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* reader grant.
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*/
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if (atomic_long_add_return(-adjustment, &sem->count) <
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RWSEM_WAITING_BIAS)
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return;
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/* Last active locker left. Retry waking readers. */
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goto try_reader_grant;
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}
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/*
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* Set it to reader-owned to give spinners an early
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* indication that readers now have the lock.
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*/
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__rwsem_set_reader_owned(sem, waiter->task);
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}
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/*
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* Grant an infinite number of read locks to the readers at the front
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* of the queue. We know that woken will be at least 1 as we accounted
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* for above. Note we increment the 'active part' of the count by the
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* number of readers before waking any processes up.
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*
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* We have to do wakeup in 2 passes to prevent the possibility that
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* the reader count may be decremented before it is incremented. It
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* is because the to-be-woken waiter may not have slept yet. So it
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* may see waiter->task got cleared, finish its critical section and
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* do an unlock before the reader count increment.
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*
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* 1) Collect the read-waiters in a separate list, count them and
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* fully increment the reader count in rwsem.
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* 2) For each waiters in the new list, clear waiter->task and
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* put them into wake_q to be woken up later.
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*/
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list_for_each_entry(waiter, &sem->wait_list, list) {
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if (waiter->type == RWSEM_WAITING_FOR_WRITE)
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break;
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woken++;
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}
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list_cut_before(&wlist, &sem->wait_list, &waiter->list);
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adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
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lockevent_cond_inc(rwsem_wake_reader, woken);
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if (list_empty(&sem->wait_list)) {
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/* hit end of list above */
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adjustment -= RWSEM_WAITING_BIAS;
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}
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if (adjustment)
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atomic_long_add(adjustment, &sem->count);
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/* 2nd pass */
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list_for_each_entry_safe(waiter, tmp, &wlist, list) {
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struct task_struct *tsk;
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tsk = waiter->task;
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get_task_struct(tsk);
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/*
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* Ensure calling get_task_struct() before setting the reader
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* waiter to nil such that rwsem_down_read_failed() cannot
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* race with do_exit() by always holding a reference count
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* to the task to wakeup.
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*/
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smp_store_release(&waiter->task, NULL);
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/*
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* Ensure issuing the wakeup (either by us or someone else)
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* after setting the reader waiter to nil.
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*/
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wake_q_add_safe(wake_q, tsk);
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}
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}
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/*
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* This function must be called with the sem->wait_lock held to prevent
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* race conditions between checking the rwsem wait list and setting the
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* sem->count accordingly.
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*/
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static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
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{
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/*
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* Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
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*/
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if (count != RWSEM_WAITING_BIAS)
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return false;
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/*
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* Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
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* are other tasks on the wait list, we need to add on WAITING_BIAS.
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*/
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count = list_is_singular(&sem->wait_list) ?
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RWSEM_ACTIVE_WRITE_BIAS :
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RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
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if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count)
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== RWSEM_WAITING_BIAS) {
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rwsem_set_owner(sem);
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return true;
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}
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return false;
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}
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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/*
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* Try to acquire write lock before the writer has been put on wait queue.
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*/
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static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
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{
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long count = atomic_long_read(&sem->count);
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while (!count || count == RWSEM_WAITING_BIAS) {
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if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
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count + RWSEM_ACTIVE_WRITE_BIAS)) {
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rwsem_set_owner(sem);
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lockevent_inc(rwsem_opt_wlock);
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return true;
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}
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}
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return false;
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}
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static inline bool owner_on_cpu(struct task_struct *owner)
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{
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/*
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* As lock holder preemption issue, we both skip spinning if
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* task is not on cpu or its cpu is preempted
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*/
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return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
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}
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static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
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{
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struct task_struct *owner;
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bool ret = true;
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BUILD_BUG_ON(!rwsem_has_anonymous_owner(RWSEM_OWNER_UNKNOWN));
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if (need_resched())
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return false;
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rcu_read_lock();
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owner = READ_ONCE(sem->owner);
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if (owner) {
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ret = is_rwsem_owner_spinnable(owner) &&
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owner_on_cpu(owner);
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}
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rcu_read_unlock();
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return ret;
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}
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/*
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* Return true only if we can still spin on the owner field of the rwsem.
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*/
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static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
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{
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struct task_struct *owner = READ_ONCE(sem->owner);
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if (!is_rwsem_owner_spinnable(owner))
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return false;
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rcu_read_lock();
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while (owner && (READ_ONCE(sem->owner) == owner)) {
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/*
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* Ensure we emit the owner->on_cpu, dereference _after_
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* checking sem->owner still matches owner, if that fails,
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* owner might point to free()d memory, if it still matches,
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* the rcu_read_lock() ensures the memory stays valid.
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*/
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barrier();
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/*
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* abort spinning when need_resched or owner is not running or
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* owner's cpu is preempted.
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*/
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if (need_resched() || !owner_on_cpu(owner)) {
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rcu_read_unlock();
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return false;
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}
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cpu_relax();
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}
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rcu_read_unlock();
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/*
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* If there is a new owner or the owner is not set, we continue
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* spinning.
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*/
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return is_rwsem_owner_spinnable(READ_ONCE(sem->owner));
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}
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static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
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{
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bool taken = false;
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preempt_disable();
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/* sem->wait_lock should not be held when doing optimistic spinning */
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if (!rwsem_can_spin_on_owner(sem))
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goto done;
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if (!osq_lock(&sem->osq))
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goto done;
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/*
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* Optimistically spin on the owner field and attempt to acquire the
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* lock whenever the owner changes. Spinning will be stopped when:
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* 1) the owning writer isn't running; or
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* 2) readers own the lock as we can't determine if they are
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* actively running or not.
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*/
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while (rwsem_spin_on_owner(sem)) {
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/*
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* Try to acquire the lock
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*/
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if (rwsem_try_write_lock_unqueued(sem)) {
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taken = true;
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break;
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}
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/*
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* When there's no owner, we might have preempted between the
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* owner acquiring the lock and setting the owner field. If
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* we're an RT task that will live-lock because we won't let
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* the owner complete.
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*/
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if (!sem->owner && (need_resched() || rt_task(current)))
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break;
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/*
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* The cpu_relax() call is a compiler barrier which forces
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* everything in this loop to be re-loaded. We don't need
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* memory barriers as we'll eventually observe the right
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* values at the cost of a few extra spins.
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*/
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cpu_relax();
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}
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osq_unlock(&sem->osq);
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done:
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preempt_enable();
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lockevent_cond_inc(rwsem_opt_fail, !taken);
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return taken;
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}
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/*
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* Return true if the rwsem has active spinner
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*/
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static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
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{
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return osq_is_locked(&sem->osq);
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}
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#else
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static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
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{
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return false;
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}
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static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
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{
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return false;
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}
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#endif
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/*
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* Wait for the read lock to be granted
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*/
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static inline struct rw_semaphore __sched *
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__rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
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{
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long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
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struct rwsem_waiter waiter;
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DEFINE_WAKE_Q(wake_q);
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waiter.task = current;
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waiter.type = RWSEM_WAITING_FOR_READ;
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raw_spin_lock_irq(&sem->wait_lock);
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if (list_empty(&sem->wait_list)) {
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/*
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* In case the wait queue is empty and the lock isn't owned
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* by a writer, this reader can exit the slowpath and return
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* immediately as its RWSEM_ACTIVE_READ_BIAS has already
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* been set in the count.
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*/
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if (atomic_long_read(&sem->count) >= 0) {
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raw_spin_unlock_irq(&sem->wait_lock);
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rwsem_set_reader_owned(sem);
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lockevent_inc(rwsem_rlock_fast);
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return sem;
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}
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adjustment += RWSEM_WAITING_BIAS;
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}
|
|
list_add_tail(&waiter.list, &sem->wait_list);
|
|
|
|
/* we're now waiting on the lock, but no longer actively locking */
|
|
count = atomic_long_add_return(adjustment, &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_WAITING_BIAS ||
|
|
(count > RWSEM_WAITING_BIAS &&
|
|
adjustment != -RWSEM_ACTIVE_READ_BIAS))
|
|
__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 */
|
|
while (true) {
|
|
set_current_state(state);
|
|
if (!waiter.task)
|
|
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);
|
|
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_add(-RWSEM_WAITING_BIAS, &sem->count);
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
__set_current_state(TASK_RUNNING);
|
|
lockevent_inc(rwsem_rlock_fail);
|
|
return ERR_PTR(-EINTR);
|
|
}
|
|
|
|
__visible struct rw_semaphore * __sched
|
|
rwsem_down_read_failed(struct rw_semaphore *sem)
|
|
{
|
|
return __rwsem_down_read_failed_common(sem, TASK_UNINTERRUPTIBLE);
|
|
}
|
|
EXPORT_SYMBOL(rwsem_down_read_failed);
|
|
|
|
__visible struct rw_semaphore * __sched
|
|
rwsem_down_read_failed_killable(struct rw_semaphore *sem)
|
|
{
|
|
return __rwsem_down_read_failed_common(sem, TASK_KILLABLE);
|
|
}
|
|
EXPORT_SYMBOL(rwsem_down_read_failed_killable);
|
|
|
|
/*
|
|
* Wait until we successfully acquire the write lock
|
|
*/
|
|
static inline struct rw_semaphore *
|
|
__rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
|
|
{
|
|
long count;
|
|
bool waiting = true; /* any queued threads before us */
|
|
struct rwsem_waiter waiter;
|
|
struct rw_semaphore *ret = sem;
|
|
DEFINE_WAKE_Q(wake_q);
|
|
|
|
/* undo write bias from down_write operation, stop active locking */
|
|
count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);
|
|
|
|
/* 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 = atomic_long_read(&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) {
|
|
__rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
|
|
/*
|
|
* The wakeup is normally called _after_ the wait_lock
|
|
* is released, but given that we are proactively waking
|
|
* readers we can deal with the wake_q overhead as it is
|
|
* similar to releasing and taking the wait_lock again
|
|
* for attempting rwsem_try_write_lock().
|
|
*/
|
|
wake_up_q(&wake_q);
|
|
|
|
/*
|
|
* Reinitialize wake_q after use.
|
|
*/
|
|
wake_q_init(&wake_q);
|
|
}
|
|
|
|
} else
|
|
count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count);
|
|
|
|
/* wait until we successfully acquire the lock */
|
|
set_current_state(state);
|
|
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 {
|
|
if (signal_pending_state(state, current))
|
|
goto out_nolock;
|
|
|
|
schedule();
|
|
lockevent_inc(rwsem_sleep_writer);
|
|
set_current_state(state);
|
|
} while ((count = atomic_long_read(&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);
|
|
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 (list_empty(&sem->wait_list))
|
|
atomic_long_add(-RWSEM_WAITING_BIAS, &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);
|
|
}
|
|
|
|
__visible struct rw_semaphore * __sched
|
|
rwsem_down_write_failed(struct rw_semaphore *sem)
|
|
{
|
|
return __rwsem_down_write_failed_common(sem, TASK_UNINTERRUPTIBLE);
|
|
}
|
|
EXPORT_SYMBOL(rwsem_down_write_failed);
|
|
|
|
__visible struct rw_semaphore * __sched
|
|
rwsem_down_write_failed_killable(struct rw_semaphore *sem)
|
|
{
|
|
return __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
|
|
}
|
|
EXPORT_SYMBOL(rwsem_down_write_failed_killable);
|
|
|
|
/*
|
|
* 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;
|
|
DEFINE_WAKE_Q(wake_q);
|
|
|
|
/*
|
|
* __rwsem_down_write_failed_common(sem)
|
|
* rwsem_optimistic_spin(sem)
|
|
* osq_unlock(sem->osq)
|
|
* ...
|
|
* atomic_long_add_return(&sem->count)
|
|
*
|
|
* - VS -
|
|
*
|
|
* __up_write()
|
|
* if (atomic_long_sub_return_release(&sem->count) < 0)
|
|
* rwsem_wake(sem)
|
|
* osq_is_locked(&sem->osq)
|
|
*
|
|
* And __up_write() must observe !osq_is_locked() when it observes the
|
|
* atomic_long_add_return() in order to not miss a wakeup.
|
|
*
|
|
* This boils down to:
|
|
*
|
|
* [S.rel] X = 1 [RmW] r0 = (Y += 0)
|
|
* MB RMB
|
|
* [RmW] Y += 1 [L] r1 = X
|
|
*
|
|
* exists (r0=1 /\ r1=0)
|
|
*/
|
|
smp_rmb();
|
|
|
|
/*
|
|
* If a spinner is present, it is not necessary to do the wakeup.
|
|
* Try to do wakeup only if the trylock succeeds to minimize
|
|
* spinlock contention which may introduce too much delay in the
|
|
* unlock operation.
|
|
*
|
|
* spinning writer up_write/up_read caller
|
|
* --------------- -----------------------
|
|
* [S] osq_unlock() [L] osq
|
|
* MB RMB
|
|
* [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
|
|
*
|
|
* Here, it is important to make sure that there won't be a missed
|
|
* wakeup while the rwsem is free and the only spinning writer goes
|
|
* to sleep without taking the rwsem. Even when the spinning writer
|
|
* is just going to break out of the waiting loop, it will still do
|
|
* a trylock in rwsem_down_write_failed() before sleeping. IOW, if
|
|
* rwsem_has_spinner() is true, it will guarantee at least one
|
|
* trylock attempt on the rwsem later on.
|
|
*/
|
|
if (rwsem_has_spinner(sem)) {
|
|
/*
|
|
* The smp_rmb() here is to make sure that the spinner
|
|
* state is consulted before reading the wait_lock.
|
|
*/
|
|
smp_rmb();
|
|
if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
|
|
return sem;
|
|
goto locked;
|
|
}
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
locked:
|
|
|
|
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;
|
|
}
|
|
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;
|
|
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;
|
|
}
|
|
EXPORT_SYMBOL(rwsem_downgrade_wake);
|