linux-kernel - Re: recursive slqb

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Message-ID: <20090302033638.GA8096@localhost>
Date:	Mon, 2 Mar 2009 11:36:38 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: recursive slqb_lock

Hi Nick,

On Sun, Mar 01, 2009 at 07:00:15PM +0200, Nick Piggin wrote:
> On Sun, Mar 01, 2009 at 04:07:25PM +0100, Nick Piggin wrote:
> > On Thu, Feb 26, 2009 at 01:34:50PM +0800, Wu Fengguang wrote:
> > > Hi Nick,
> > > 
> > > I got a lockdep warning. It looks like the slqb_lock will be taken
> > > twice in the call chain:
> > > 
> > >         s_start()  => take slqb_lock
> > >           s_show()
> > >             gather_stats()  => take slqb_lock again
> > 
> > Hey, thanks for this. The following patch should fix it.
> > 
> > --
> > Fix the lockdep error reported by Fengguang where down_read slqb_lock
> > is taken twice, with the possibility for a deadlock.
> > 
> > Signed-off-by: Nick Piggin <npiggin@...e.de>

> > +static void gather_stats(struct kmem_cache *s, struct stats_gather *stats)
> > +{
> > +	down_read(&slqb_lock); /* hold off hotplug */
> > +	gather_stats(s, stats);
> > +	up_read(&slqb_lock);
> > +}
> 
> Oops, I didn't test it. That call should be to gather_stats_locked of
> course.

Yeah it fixed the lockdep warning :)

Thanks,
Fengguang
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