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SubjectRe: [PATCH] atomic: Only take lock when the counter drops to zero on UP as well
On Tue, Apr 14, 2009 at 08:52:39AM +0200, Nick Piggin wrote:
> On Fri, Apr 10, 2009 at 06:13:57PM +0200, Jan Blunck wrote:
> > I think it is wrong to unconditionally take the lock before calling
> > atomic_dec_and_test() in _atomic_dec_and_lock(). This will deadlock in
> > situation where it is known that the counter will not reach zero (e.g. holding
> > another reference to the same object) but the lock is already taken.
> >
> > Signed-off-by: Jan Blunck <jblunck@suse.de>
>
> Paul's worry about callers aside, I think it is probably a good idea
> to reduce ifdefs and share more code.

I am also OK with this patch.

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> So for this patch,
>
> Acked-by: Nick Piggin <npiggin@suse.de>
>
> > ---
> > lib/dec_and_lock.c | 3 +--
> > 1 files changed, 1 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/dec_and_lock.c b/lib/dec_and_lock.c
> > index a65c314..e73822a 100644
> > --- a/lib/dec_and_lock.c
> > +++ b/lib/dec_and_lock.c
> > @@ -19,11 +19,10 @@
> > */
> > int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
> > {
> > -#ifdef CONFIG_SMP
> > /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
> > if (atomic_add_unless(atomic, -1, 1))
> > return 0;
> > -#endif
> > +
> > /* Otherwise do it the slow way */
> > spin_lock(lock);
> > if (atomic_dec_and_test(atomic))
> > --
> > 1.6.0.2


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