mm, oom: reduce dependency on tasklist_lock

Since exiting tasks require write_lock_irq(&tasklist_lock) several times,
try to reduce the amount of time the readside is held for oom kills.  This
makes the interface with the memcg oom handler more consistent since it
now never needs to take tasklist_lock unnecessarily.

The only time the oom killer now takes tasklist_lock is when iterating the
children of the selected task, everything else is protected by
rcu_read_lock().

This requires that a reference to the selected process, p, is grabbed
before calling oom_kill_process().  It may release it and grab a reference
on another one of p's threads if !p->mm, but it also guarantees that it
will release the reference before returning.

[hughd@google.com: fix duplicate put_task_struct()]
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 0 insertions, 3 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b78972e2f43f..77a29cea5d76 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1521,11 +1521,8 @@ void __mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
         if (!chosen)
                 return;
         points = chosen_points * 1000 / totalpages;
-        read_lock(&tasklist_lock);
         oom_kill_process(chosen, gfp_mask, order, points, totalpages, memcg,
                          NULL, "Memory cgroup out of memory");
-        read_unlock(&tasklist_lock);
-        put_task_struct(chosen);
 }
 
 static unsigned long mem_cgroup_reclaim(struct mem_cgroup *memcg,