[PATCH] zcache: avoid AB-BA deadlock condition

[Andrea Righi]

On Mon, Feb 27, 2012 at 09:23:24AM -0800, Dan Magenheimer wrote:
> > From: Andrea Righi [mailto:andrea at betterlinux.com]
> > Sent: Monday, February 20, 2012 5:12 AM
> > To: Greg Kroah-Hartman
> > Cc: Dan Magenheimer; Seth Jennings; devel at driverdev.osuosl.org; linux-kernel at vger.kernel.org;
> > stable at kernel.org
> > Subject: [PATCH] zcache: avoid AB-BA deadlock condition
> > 
> > Commit 9256a47 fixed a deadlock condition, being sure that the buddy
> > list spinlock is always taken before the page spinlock.
> > 
> > However in zbud_free_and_delist() locking order is the opposite
> > (page lock -> list lock).
> > 
> > Possible unsafe locking scenario (reported by lockdep):
> > 
> >         CPU0                    CPU1
> >         ----                    ----
> >    lock(&(&zbpg->lock)->rlock);
> >                                 lock(zbud_budlists_spinlock);
> >                                 lock(&(&zbpg->lock)->rlock);
> >    lock(zbud_budlists_spinlock);
> > 
> > Fix by grabbing the locks in opposite order in zbud_free_and_delist().
> > 
> > Signed-off-by: Andrea Righi <andrea at betterlinux.com>
> 
> Acked-by: Dan Magenheimer <dan.magenheimer at oracle.com>
> 
> Thanks for catching this Andrea!  (And thanks also to
> Alex Vallacis-Lasso for independently reporting and testing:
> http://permalink.gmane.org/gmane.linux.kernel/1257214 )
> 
> Greg, this patch could be targeted for 3.3-rc6 and 3.2-stable.
> AFAIK, nobody has actually experienced a deadlock from this so
> if Linus has the screws down tight for -rc6, it could wait
> until the 3.4 window.

If it helps, without the fix I can easily trigger the lockdep splat
running a echo 3 > /proc/sys/vm/drop_caches.

-Andrea