oom: fix constraint deadlock

Fixes a deadlock in the OOM killer for allocations that are not
__GFP_HARDWALL.

Before the OOM killer checks for the allocation constraint, it takes
callback_mutex.

constrained_alloc() iterates through each zone in the allocation zonelist
and calls cpuset_zone_allowed_softwall() to determine whether an allocation
for gfp_mask is possible.  If a zone's node is not in the OOM-triggering
task's mems_allowed, it is not exiting, and we did not fail on a
__GFP_HARDWALL allocation, cpuset_zone_allowed_softwall() attempts to take
callback_mutex to check the nearest exclusive ancestor of current's cpuset.
 This results in deadlock.

We now take callback_mutex after iterating through the zonelist since we
don't need it yet.

Cc: Andi Kleen <ak@suse.de>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Christoph Lameter <clameter@engr.sgi.com>
Cc: Martin J. Bligh <mbligh@mbligh.org>
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -399,6 +399,7 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask, int order)
 	struct task_struct *p;
 	unsigned long points = 0;
 	unsigned long freed = 0;
+	int constraint;
 
 	blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
 	if (freed > 0)
@@ -416,14 +417,15 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask, int order)
 	if (sysctl_panic_on_oom == 2)
 		panic("out of memory. Compulsory panic_on_oom is selected.\n");
 
-	cpuset_lock();
-	read_lock(&tasklist_lock);
-
 	/*
 	 * Check if there were limitations on the allocation (only relevant for
 	 * NUMA) that may require different handling.
 	 */
-	switch (constrained_alloc(zonelist, gfp_mask)) {
+	constraint = constrained_alloc(zonelist, gfp_mask);
+	cpuset_lock();
+	read_lock(&tasklist_lock);
+
+	switch (constraint) {
 	case CONSTRAINT_MEMORY_POLICY:
 		oom_kill_process(current, points,
 				"No available memory (MPOL_BIND)");