Re: [PATCH] atomic: Fix _atomic_dec_and_lock() deadlock on UP

From: Andrew Morton
Date: Mon Jun 15 2009 - 15:32:21 EST


On Mon, 15 Jun 2009 15:12:23 -0400
Valerie Aurora <vaurora@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxx>
> > Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Acked-by: Nick Piggin <npiggin@xxxxxxx>
> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >
> >
> > 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..

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/