linux-kernel - Re: [PATCH] mm/memcontrol: avoid a spurious gcc warning

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Date:	Mon, 25 Jan 2016 13:55:38 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Michal Hocko <mhocko@...nel.org>,
	Vladimir Davydov <vdavydov@...tuozzo.com>,
	linux-arm-kernel@...ts.infradead.org, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] mm/memcontrol: avoid a spurious gcc warning

Hi Arnd,

On Mon, Jan 25, 2016 at 04:45:50PM +0100, Arnd Bergmann wrote:
> When CONFIG_DEBUG_VM is set, the various VM_BUG_ON() confuse gcc to
> the point where it cannot remember that 'memcg' is known to be initialized:
> 
> mm/memcontrol.c: In function 'mem_cgroup_can_attach':
> mm/memcontrol.c:4791:9: warning: 'memcg' may be used uninitialized in this function [-Wmaybe-uninitialized]
> 
> On ARM gcc-5.1, the above happens when any two or more of the VM_BUG_ON()
> are active, but not when I remove most or all of them. This is clearly
> random behavior and the only way I've found to shut up the warning is
> to add an explicit initialization.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Thanks Arnd.

This has been fixed upstream already:

commit eed67d75b66748a498a0592d9704081a98509444
Author: Ross Zwisler <ross.zwisler@...ux.intel.com>
Date:   Wed Dec 23 14:53:27 2015 -0700

    cgroup: Fix uninitialized variable warning
    
    Commit 1f7dd3e5a6e4 ("cgroup: fix handling of multi-destination migration
    from subtree_control enabling") introduced the following compiler warning:
    
    mm/memcontrol.c: In function ‘mem_cgroup_can_attach’:
    mm/memcontrol.c:4790:9: warning: ‘memcg’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       mc.to = memcg;
             ^
    
    Fix this by initializing 'memcg' to NULL.
    
    This was found using gcc (GCC) 4.9.2 20150212 (Red Hat 4.9.2-6).
    
    Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
    Signed-off-by: Tejun Heo <tj@...nel.org>