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    SubjectRe: [PATCH] atomic: Fix _atomic_dec_and_lock() deadlock on UP
    On Mon, 15 Jun 2009 14:11:13 -0400
    Valerie Aurora <> wrote:

    > _atomic_dec_and_lock() can deadlock on UP with spinlock debugging
    > enabled. Currently, on UP we unconditionally spin_lock() first, which
    > calls __spin_lock_debug(), which takes the lock unconditionally even
    > on UP. This will deadlock in situations in which we call
    > atomic_dec_and_lock() knowing that the counter won't go to zero
    > (because we hold another reference) and that we already hold the lock.
    > Instead, we should use the SMP code path which only takes the lock if
    > necessary.

    Yup, I have this queued for 2.6.31 as
    with a different changelog:

    _atomic_dec_and_lock() should not unconditionally take the lock before
    calling atomic_dec_and_test() in the UP case. For consistency reasons it
    should behave exactly like in the SMP case.

    Besides that this works around the problem that with CONFIG_DEBUG_SPINLOCK
    this spins in __spin_lock_debug() if the lock is already taken even if the
    counter doesn't drop to 0.

    Signed-off-by: Jan Blunck <>
    Acked-by: Paul E. McKenney <>
    Acked-by: Nick Piggin <>
    Signed-off-by: Andrew Morton <>

    I can't remember why we decided that 2.6.30 doesn't need this.

     \ /
      Last update: 2009-06-15 20:49    [W:0.023 / U:9.748 seconds]
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