Messages in this thread | | | Subject | Re: [PATCH 1/3] ipc/sem.c: Chance memory barrier in sem_lock() to smp_rmb() | From | Davidlohr Bueso <> | Date | Thu, 09 Oct 2014 16:07:34 -0700 |
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On Mon, 2014-10-06 at 20:32 +0200, Manfred Spraul wrote: > When I fixed bugs in the sem_lock() logic, I was more conservative than > necessary. > Therefore it is safe to replace the smp_mb() with smp_rmb(). > And: With smp_rmb(), semop() syscalls are up to 10% faster. > > The race we must protect against is: > > sem->lock is free > sma->complex_count = 0 > sma->sem_perm.lock held by thread B > > thread A: > > A: spin_lock(&sem->lock) > > B: sma->complex_count++; (now 1) > B: spin_unlock(&sma->sem_perm.lock); > > A: spin_is_locked(&sma->sem_perm.lock); > A: XXXXX memory barrier > A: if (sma->complex_count == 0) > > Thread A must read the increased complex_count value, i.e. the read must > not be reordered with the read of sem_perm.lock done by spin_is_locked(). > > Since it's about ordering of reads, smp_rmb() is sufficient. > > Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
With a suggestion below.
> --- > ipc/sem.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/ipc/sem.c b/ipc/sem.c > index 454f6c6..ffc71de 100644 > --- a/ipc/sem.c > +++ b/ipc/sem.c > @@ -326,10 +326,16 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops, > > /* Then check that the global lock is free */ > if (!spin_is_locked(&sma->sem_perm.lock)) { > - /* spin_is_locked() is not a memory barrier */ > - smp_mb(); > + /* > + * The next test must happen after the test for > + * sem_perm.lock, otherwise we can race with another > + * thread that does > + * complex_count++;spin_unlock(sem_perm.lock); > + */
How about this comment instead:
/* * The ipc object lock check must be visible on all cores before * rechecking the complex count. Otherwise we can race with * another thread that does: * complex_count++++; * spin_unlock(sem_perm.lock); */
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