linux-kernel - Re: [PATCH] slub: Fix sysfs circular locking dependency

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Message-ID: <AANLkTinCZzJWYGnxXq_Nwv7QSkEV4Tg-te-1y0PCeRRY@mail.gmail.com>
Date:	Wed, 5 Jan 2011 19:50:49 +0100
From:	Bart Van Assche <bvanassche@....org>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Christoph Lameter <cl@...ux.com>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] slub: Fix sysfs circular locking dependency

On Wed, Jan 5, 2011 at 7:26 PM, Pekka Enberg <penberg@...nel.org> wrote:
>
> On Wed, Jan 5, 2011 at 7:10 PM, Christoph Lameter <cl@...ux.com> wrote:
> > How about this solution? In the common case of kernels without hotplug
> > support there will be no locking at all.
> >
> >
> > Subject: slub: Avoid use of slub_lock in show_slab_objects()
> >
> > The purpose of the locking is to prevent removal and additions
> > of nodes when statistics are gathered for a slab cache. So we
> > need to avoid racing with memory hotplug functionality.
> >
> > It is enough to take the memory hotplug locks there instead
> > of the slub_lock.
> >
> >
> > Signed-off-by: Christoph Lameter <cl@...ux.com>
> >
> > ---
> >  mm/slub.c |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > Index: linux-2.6/mm/slub.c
> > ===================================================================
> > --- linux-2.6.orig/mm/slub.c    2011-01-05 09:55:34.000000000 -0600
> > +++ linux-2.6/mm/slub.c 2011-01-05 09:56:27.000000000 -0600
> > @@ -3821,7 +3821,7 @@ static ssize_t show_slab_objects(struct
> >                }
> >        }
> >
> > -       down_read(&slub_lock);
> > +       lock_memory_hotplug();
> >  #ifdef CONFIG_SLUB_DEBUG
> >        if (flags & SO_ALL) {
> >                for_each_node_state(node, N_NORMAL_MEMORY) {
> > @@ -3862,7 +3862,7 @@ static ssize_t show_slab_objects(struct
> >                        x += sprintf(buf + x, " N%d=%lu",
> >                                        node, nodes[node]);
> >  #endif
> > -       up_read(&slub_lock);
> > +       unlock_memory_hotplug();
> >        kfree(nodes);
> >        return x + sprintf(buf + x, "\n");
> >  }
>
> Makes sense. Bart, does this fix the problem for you?

The action sequence that had triggered this sequence (invoke
kmem_cache_create(); read all files in /sys/kernel/slab; invoke
kmem_cache_destroy()) does now pass without triggering lock inversion
complaints.

Bart.
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