linux-kernel - Re: [PATCH] atomic: Fix _atomic_dec_and

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Message-Id: <20090615114543.80c420b3.akpm@linux-foundation.org>
Date:	Mon, 15 Jun 2009 11:45:43 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	npiggin@...e.de, jblunck@...e.de, linux-kernel@...r.kernel.org,
	paulmck@...ibm.com
Subject: Re: [PATCH] atomic: Fix _atomic_dec_and_lock() deadlock on UP

On Mon, 15 Jun 2009 14:11:13 -0400
Valerie Aurora <vaurora@...hat.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@...e.de>
  Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
  Acked-by: Nick Piggin <npiggin@...e.de>
  Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>

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

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