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    Subject[tip:core/urgent] mutex: Fix optimistic spinning vs. BKL
    Commit-ID:  fd6be105b883244127a734ac9f14ae94a022dcc0
    Gitweb: http://git.kernel.org/tip/fd6be105b883244127a734ac9f14ae94a022dcc0
    Author: Tony Breeds <tony@bakeyournoodle.com>
    AuthorDate: Wed, 19 May 2010 15:46:36 +1000
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Wed, 19 May 2010 08:18:44 +0200

    mutex: Fix optimistic spinning vs. BKL

    Currently, we can hit a nasty case with optimistic
    spinning on mutexes:

    CPU A tries to take a mutex, while holding the BKL

    CPU B tried to take the BLK while holding the mutex

    This looks like a AB-BA scenario but in practice, is
    allowed and happens due to the auto-release on
    schedule() nature of the BKL.

    In that case, the optimistic spinning code can get us
    into a situation where instead of going to sleep, A
    will spin waiting for B who is spinning waiting for
    A, and the only way out of that loop is the
    need_resched() test in mutex_spin_on_owner().

    This patch fixes it by completely disabling spinning
    if we own the BKL. This adds one more detail to the
    extensive list of reasons why it's a bad idea for
    kernel code to be holding the BKL.

    Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
    Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
    Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: <stable@kernel.org>
    LKML-Reference: <20100519054636.GC12389@ozlabs.org>
    [ added an unlikely() attribute to the branch ]
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    kernel/mutex.c | 7 +++++++
    1 files changed, 7 insertions(+), 0 deletions(-)

    diff --git a/kernel/mutex.c b/kernel/mutex.c
    index 632f04c..4c0b7b3 100644
    --- a/kernel/mutex.c
    +++ b/kernel/mutex.c
    @@ -172,6 +172,13 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
    struct thread_info *owner;

    /*
    + * If we own the BKL, then don't spin. The owner of
    + * the mutex might be waiting on us to release the BKL.
    + */
    + if (unlikely(current->lock_depth >= 0))
    + break;
    +
    + /*
    * If there's an owner, wait for it to either
    * release the lock or go to sleep.
    */

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