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    Subject[tip:perfcounters/core] mutex: add atomic_dec_and_mutex_lock()
    Commit-ID:  3a2d367d9aabac486ac4444c6c7ec7a1dab16267
    Gitweb: http://git.kernel.org/tip/3a2d367d9aabac486ac4444c6c7ec7a1dab16267
    Author: Eric Paris <eparis@redhat.com>
    AuthorDate: Mon, 23 Mar 2009 18:22:09 +0100
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Mon, 23 Mar 2009 21:45:10 +0100

    mutex: add atomic_dec_and_mutex_lock()

    Much like the atomic_dec_and_lock() function in which we take an hold a
    spin_lock if we drop the atomic to 0 this function takes and holds the
    mutex if we dec the atomic to 0.

    Signed-off-by: Eric Paris <eparis@redhat.com>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Paul Mackerras <paulus@samba.org>
    LKML-Reference: <20090323172417.410913479@chello.nl>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>


    ---
    include/linux/mutex.h | 23 +++++++++++++++++++++++
    1 files changed, 23 insertions(+), 0 deletions(-)

    diff --git a/include/linux/mutex.h b/include/linux/mutex.h
    index 3069ec7..93054fc 100644
    --- a/include/linux/mutex.h
    +++ b/include/linux/mutex.h
    @@ -151,4 +151,27 @@ extern int __must_check mutex_lock_killable(struct mutex *lock);
    extern int mutex_trylock(struct mutex *lock);
    extern void mutex_unlock(struct mutex *lock);

    +/**
    + * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
    + * @cnt: the atomic which we are to dec
    + * @lock: the mutex to return holding if we dec to 0
    + *
    + * return true and hold lock if we dec to 0, return false otherwise
    + */
    +static inline int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
    +{
    + /* dec if we can't possibly hit 0 */
    + if (atomic_add_unless(cnt, -1, 1))
    + return 0;
    + /* we might hit 0, so take the lock */
    + mutex_lock(lock);
    + if (!atomic_dec_and_test(cnt)) {
    + /* when we actually did the dec, we didn't hit 0 */
    + mutex_unlock(lock);
    + return 0;
    + }
    + /* we hit 0, and we hold the lock */
    + return 1;
    +}
    +
    #endif

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