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    Subject[PATCH 3.12 102/122] ipc/sem.c: change memory barrier in sem_lock() to smp_rmb()
    Date
    From: Manfred Spraul <manfred@colorfullife.com>

    3.12-stable review patch. If anyone has any objections, please let me know.

    ===============

    commit 2e094abfd1f29a08a60523b42d4508281b8dee0e upstream.

    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.

    [akpm@linux-foundation.org: update sem_lock() comment, from Davidlohr]
    Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
    Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
    Acked-by: Rafael Aquini <aquini@redhat.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    ---
    ipc/sem.c | 13 ++++++++++---
    1 file changed, 10 insertions(+), 3 deletions(-)

    diff --git a/ipc/sem.c b/ipc/sem.c
    index db9d241af133..0c312ac04e49 100644
    --- a/ipc/sem.c
    +++ b/ipc/sem.c
    @@ -326,10 +326,17 @@ 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 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);
    + */
    + 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.
    */
    --
    2.2.2


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