Date: Sat, 19 Dec 2015 11:51:38 +0300
From: Vladimir Davydov <vdavydov@virtuozzo.com>
To: Andrew Morton <akpm@linux-foundation.org>
CC: Johannes Weiner <hannes@cmpxchg.org>, <stable@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] mm: memcontrol: fix possible memcg leak due to
 interrupted reclaim
Message-ID: <20151219085137.GV28521@esperanza>
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 <20151217150217.a02c264ce9b5335b02bae888@linux-foundation.org>
 <20151218153202.GS28521@esperanza>
 <20151218160041.GA4201@cmpxchg.org>
 <20151218162405.GU28521@esperanza>
 <20151218144004.6ec6189817b64e04d9405001@linux-foundation.org>
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On Fri, Dec 18, 2015 at 02:40:04PM -0800, Andrew Morton wrote:
> On Fri, 18 Dec 2015 19:24:05 +0300 Vladimir Davydov <vdavydov@virtuozzo.com> wrote:
> 
> > 
> > OK, got it, thanks. Here goes the incremental patch (it should also fix
> > the warning regarding unused cmpxchg returned value):
> > ---
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index fc25dc211eaf..908c075e04eb 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -864,7 +864,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> >  			 * might block it. So we clear iter->position right
> >  			 * away.
> >  			 */
> > -			cmpxchg(&iter->position, pos, NULL);
> > +			(void)cmpxchg(&iter->position, pos, NULL);
> 
> No, this doesn't actually squish the __must_check warning.

The warning was caused not by a __must_check annotation - using it for
cmpxchg would be just wrong - it was caused by type conversion done in
expansion of cmpxchg macro:

   arch/m68k/include/asm/cmpxchg.h:121:3: warning: value computed is not used [-Wunused-value]
     ((__typeof__(*(ptr)))__cmpxchg((ptr), (unsigned long)(o),     \
      ^

(see http://www.spinics.net/lists/linux-mm/msg99133.html)

Type conversion to (void) helps suppressing this warning, and it seems
this is what is done commonly (e.g. see kernel/rcu/tree_plugin.h)

Thanks,
Vladimir
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