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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 <vaurora@redhat.com> 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
atomic-only-take-lock-when-the-counter-drops-to-zero-on-up-as-well.patch,
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 <jblunck@suse.de>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>


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



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