Messages in this thread | | | Date | Mon, 15 Jun 2009 15:12:23 -0400 | From | Valerie Aurora <> | Subject | Re: [PATCH] atomic: Fix _atomic_dec_and_lock() deadlock on UP |
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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@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.
Great, last I heard the changelog was still a problem. Thanks,
-VAL
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