Hi David,

On Wed, Mar 25, 2015 at 08:31:49PM -0700, David Rientjes wrote:

On Wed, 25 Mar 2015, Johannes Weiner wrote:

quoted

exit_oom_victim() already knows that TIF_MEMDIE is set, and nobody
else can clear it concurrently.  Use clear_thread_flag() directly.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

For the oom killer, that's true because of task_lock(): we always only set 
TIF_MEMDIE when there is a valid p->mm and it's cleared in the exit path 
after the unlock, acting as a barrier, when p->mm is set to NULL so it's 
no longer a valid victim.  So that part is fine.

The problem is the android low memory killer that does 
mark_tsk_oom_victim() without the protection of task_lock(), it's just rcu 
protected so the reference to the task itself is guaranteed to still be 
valid.

But this is about *setting* it without a lock.  My point was that once
TIF_MEMDIE is actually set, the task owns it and nobody else can clear
it for them, so it's safe to test and clear non-atomically from the
task's own context.  Am I missing something?

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