unimore/kernel-hacking-2024-linux-steffo
unimore/kernel-hacking-2024-linux-steffo
1
Fork 0
Code Pull requests 5 Wiki Activity

rwsem: do not block readers at head of queue if other readers are active

Browse source 
This change fixes a race condition where a reader might determine it
needs to block, but by the time it acquires the wait_lock the rwsem has
active readers and no queued waiters.

In this situation the reader can run in parallel with the existing
active readers; it does not need to block until the active readers
complete.

Thanks to Peter Hurley for noticing this possible race.

Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Michel Lespinasse 2013-05-07 06:46:00 -07:00 committed by Linus Torvalds
parent fe6e674c61
commit 25c3932596
1 changed files with 8 additions and 2 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

10
lib/rwsem.c
View file

@ -163,8 +163,14 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
/* we're now waiting on the lock, but no longer actively locking */ /* we're now waiting on the lock, but no longer actively locking */
count = rwsem_atomic_update(adjustment, sem); count = rwsem_atomic_update(adjustment, sem);
/* If there are no active locks, wake the front queued process(es). */ /* If there are no active locks, wake the front queued process(es).
if (!(count & RWSEM_ACTIVE_MASK)) *
* 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); sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
raw_spin_unlock_irq(&sem->wait_lock); raw_spin_unlock_irq(&sem->wait_lock);