[PATCH 8/9] oom: make oom_reaper freezable

Michal Hocko
2016-03-22T11:01:31Z

From: Michal Hocko <mhocko@suse.com>

After "oom: clear TIF_MEMDIE after oom_reaper managed to unmap the
address space" oom_reaper will call exit_oom_victim on the target
task after it is done. This might however race with the PM freezer:

CPU0 CPU1 CPU2
freeze_processes
try_to_freeze_tasks
# Allocation request
out_of_memory
oom_killer_disable
wake_oom_reaper(P1)
__oom_reap_task
exit_oom_victim(P1)
wait_event(oom_victims==0)
[...]
do_exit(P1)
perform IO/interfere with the freezer

which breaks the oom_killer_disable semantic. We no longer have a
guarantee that the oom victim won't interfere with the freezer because
it might be anywhere on the way to do_exit while the freezer thinks the
task has already terminated. It might trigger IO or touch devices which
are frozen already.

In order to close this race, make the oom_reaper thread freezable. This
will work because
a) already running oom_reaper will block freezer to enter the
quiescent state
b) wake_oom_reaper will not wake up the reaper after it has been
frozen
c) the only way to call exit_oom_victim after try_to_freeze_tasks
is from the oom victim's context when we know the further
interference shouldn't be possible

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
mm/oom_kill.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index af75260f32c3..bed2885d10b0 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -524,6 +524,8 @@ static void oom_reap_task(struct task_struct *tsk)

static int oom_reaper(void *unused)
{
+ set_freezable();
+
while (true) {
struct task_struct *tsk = NULL;

--
2.7.0