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locking/rwsem: Document barrier need when waking tasks 

The need for the smp_mb() in __rwsem_do_wake() should be
properly documented. Applies to both xadd and spinlock
variants.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michel Lespinasse <walken@google.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Link: http://lkml.kernel.org/r/1422609267-15102-3-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Davidlohr Bueso 2015-01-30 01:14:24 -08:00 committed by Ingo Molnar
parent a212946446
commit 49e4b2bcf7
2 changed files with 14 additions and 0 deletions
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7
kernel/locking/rwsem-spinlock.c
View File

@ -85,6 +85,13 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
list_del(&waiter->list);
tsk = waiter->task;
/*
* Make sure we do not wakeup the next reader before
* setting the nil condition to grant the next reader;
* otherwise we could miss the wakeup on the other
* side and end up sleeping again. See the pairing
* in rwsem_down_read_failed().
*/
smp_mb();
waiter->task = NULL;
wake_up_process(tsk);

7
kernel/locking/rwsem-xadd.c
View File

@ -186,6 +186,13 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
waiter = list_entry(next, struct rwsem_waiter, list);
next = waiter->list.next;
tsk = waiter->task;
/*
* Make sure we do not wakeup the next reader before
* setting the nil condition to grant the next reader;
* otherwise we could miss the wakeup on the other
* side and end up sleeping again. See the pairing
* in rwsem_down_read_failed().
*/
smp_mb();
waiter->task = NULL;
wake_up_process(tsk);