linux-kernel - Re: lockdep complaints in slab allocator

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Message-ID: <20091130161845.GA8338@linux.vnet.ibm.com>
Date:	Mon, 30 Nov 2009 08:18:45 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org,
	cl@...ux-foundation.org, mpm@...enic.com,
	LKML <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: lockdep complaints in slab allocator

On Tue, Nov 24, 2009 at 01:47:40PM -0800, Paul E. McKenney wrote:
> On Tue, Nov 24, 2009 at 10:59:44PM +0200, Pekka Enberg wrote:
> > On Tue, Nov 24, 2009 at 6:23 PM, Paul E. McKenney
> > <paulmck@...ux.vnet.ibm.com> wrote:
> > > On Mon, Nov 23, 2009 at 09:00:00PM +0200, Pekka Enberg wrote:
> > >> Hi Peter,
> > >>
> > >> On Fri, 2009-11-20 at 16:09 +0100, Peter Zijlstra wrote:
> > >> > > Uh, ok, so apparently I was right after all. There's a comment in
> > >> > > free_block() above the slab_destroy() call that refers to the comment
> > >> > > above alloc_slabmgmt() function definition which explains it all.
> > >> > >
> > >> > > Long story short: ->slab_cachep never points to the same kmalloc cache
> > >> > > we're allocating or freeing from. Where do we need to put the
> > >> > > spin_lock_nested() annotation? Would it be enough to just use it in
> > >> > > cache_free_alien() for alien->lock or do we need it in
> > >> > > cache_flusharray() as well?
> > >> >
> > >> > You'd have to somehow push the nested state down from the
> > >> > kmem_cache_free() call in slab_destroy() to all nc->lock sites below.
> > >>
> > >> That turns out to be _very_ hard. How about something like the following
> > >> untested patch which delays slab_destroy() while we're under nc->lock.
> > >>
> > >>                       Pekka
> > >
> > > Preliminary tests look good!  The test was a ten-hour rcutorture run on
> > > an 8-CPU Power system with a half-second delay between randomly chosen
> > > CPU-hotplug operations.  No lockdep warnings.  ;-)
> > >
> > > Will keep hammering on it.
> > 
> > Thanks! Please let me know when you're hammered it enough :-). Peter,
> > may I have your ACK or NAK on the patch, please?
> 
> I expect to hammer it over the USA Thanksgiving holiday Thu-Sun this week.
> It is like this, Pekka: since I don't drink, it is instead your code
> that is going to get hammered this weekend!

And the runs with your patch were free from lockdep complaints, while
those runs lacking your patch did raise lockdep's ire.  So feel free to
add "Tested-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>" to the
patch.

And thank you for the fix!!!

							Thanx, Paul
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