[PATCH] mm/memcontrol: avoid a spurious gcc warning

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@arndb.de>
---
 mm/memcontrol.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d06cae2de783..9340eb981653 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -4762,6 +4762,7 @@ static int mem_cgroup_can_attach(struct cgroup_taskset *tset)
 	 * multiple.
 	 */
 	p = NULL;
+	memcg = NULL;
 	cgroup_taskset_for_each_leader(leader, css, tset) {
 		WARN_ON_ONCE(p);
 		p = leader;

-- 
2.7.0

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@arndb.de>

Thanks Arnd.

This has been fixed upstream already:

commit eed67d75b66748a498a0592d9704081a98509444
Author: Ross Zwisler [off-list ref]
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 [off-list ref]
    Signed-off-by: Tejun Heo [off-list ref]