Messages in this thread | | | Date | Tue, 27 Jan 2015 18:07:32 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/6] locking/rwsem: Document barrier need when waking tasks |
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On Sun, Jan 25, 2015 at 11:36:05PM -0800, Davidlohr Bueso wrote: > 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> > --- > kernel/locking/rwsem-spinlock.c | 5 +++++ > kernel/locking/rwsem-xadd.c | 5 +++++ > 2 files changed, 10 insertions(+) > > diff --git a/kernel/locking/rwsem-spinlock.c b/kernel/locking/rwsem-spinlock.c > index 2555ae1..54f7a17 100644 > --- a/kernel/locking/rwsem-spinlock.c > +++ b/kernel/locking/rwsem-spinlock.c > @@ -85,6 +85,11 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite) > > list_del(&waiter->list); > tsk = waiter->task; > + /* > + * Ensure that all cores see the read before > + * setting it to the waiter task to nil, as that > + * grants the read lock to the next task. > + */ > smp_mb(); > waiter->task = NULL; > wake_up_process(tsk);
Could you enhance that comment by pointing at the pairing code? Is that the wait loop in rwsem_down_read_failed()?
Also, the comment confuses, how can all cores observe a read into a local variable?
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