[PATCH] mm, oom_reaper: do not use siglock in try_oom_reaper

[Michal Hocko <mhocko@kernel.org>]

From: Michal Hocko <mhocko@suse.com>

Oleg has noted that siglock usage in try_oom_reaper is both pointless
and dangerous. signal_group_exit can be checked lockless. The problem
is that sighand becomes NULL in __exit_signal so we can crash.

Fixes: 3ef22dfff239 ("oom, oom_reaper: try to reap tasks which skip regular OOM killer path")
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
Hi Andrew,
Oleg has noticed this while reviewing http://lkml.kernel.org/r/20160530173505.GA25287@redhat.com
this should go in 4.7.

 mm/oom_kill.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index e01cc3e2e755..25eac62c190c 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -625,8 +625,6 @@ void try_oom_reaper(struct task_struct *tsk)
 	if (atomic_read(&mm->mm_users) > 1) {
 		rcu_read_lock();
 		for_each_process(p) {
-			bool exiting;
-
 			if (!process_shares_mm(p, mm))
 				continue;
 			if (fatal_signal_pending(p))
@@ -636,10 +634,7 @@ void try_oom_reaper(struct task_struct *tsk)
 			 * If the task is exiting make sure the whole thread group
 			 * is exiting and cannot acces mm anymore.
 			 */
-			spin_lock_irq(&p->sighand->siglock);
-			exiting = signal_group_exit(p->signal);
-			spin_unlock_irq(&p->sighand->siglock);
-			if (exiting)
+			if (signal_group_exit(p->signal))
 				continue;
 
 			/* Give up */
-- 
2.8.1

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