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

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Date:	Mon, 15 Jun 2009 12:31:44 -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 15:12:23 -0400
Valerie Aurora <vaurora@...hat.com> wrote:

> On Mon, Jun 15, 2009 at 11:45:43AM -0700, Andrew Morton wrote:
> > 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.
> 
> Great, last I heard the changelog was still a problem.  Thanks,
> 

<goes back and checks>

OK, I decided that we didn't need this in 2.6.30 or earlier because
Jan's union mount code is the only known triggerer of the problem.

However the patch is clearly a suitable thing for -stable.  Opinions
are sought..

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