[patch -mm 1/2] oom: protect oom_disable_count with task_lock in fork

[David Rientjes <rientjes@google.com>]

task_lock(p) protects p->mm->oom_disable_count such that it accurately 
represents the number of threads attached to that mm that cannot be
killed by the oom killer.  p->signal->oom_score_adj is never changed
without holding the lock.

This was missed in the fork() path, so we take the lock to ensure
checking its oom_score_adj and decrementing oom_disable_count don't race.

Reported-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: David Rientjes <rientjes@google.com>
---
 kernel/fork.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1304,8 +1304,10 @@ bad_fork_cleanup_namespaces:
 	exit_task_namespaces(p);
 bad_fork_cleanup_mm:
 	if (p->mm) {
+		task_lock(p);
 		if (p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
 			atomic_dec(&p->mm->oom_disable_count);
+		task_unlock(p);
 		mmput(p->mm);
 	}
 bad_fork_cleanup_signal:

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