• Tony Breeds's avatar
    mutex: Fix optimistic spinning vs. BKL · fd6be105
    Tony Breeds authored
    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: default avatarTony Breeds <tony@bakeyournoodle.com>
    Acked-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Acked-by: default avatarPeter 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: default avatarIngo Molnar <mingo@elte.hu>
    fd6be105
mutex.c 13.4 KB