On Mon, Oct 06, 2014 at 08:32:41PM +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 <manf...@colorfullife.com>
> ---
> 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);
> + */
> + smp_rmb();
>
> - /* Now repeat the test of complex_count:
> + /*
> + * Now repeat the test of complex_count:
> * It can't change anymore until we drop sem->lock.
> * Thus: if is now 0, then it will stay 0.
> */
> --
> 1.9.3
>
Acked-by: Rafael Aquini <aqu...@redhat.com>
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