Re: [patch 03/12] mm: oom_kill: switch test-and-clear of known TIF_MEMDIE to clear

[Johannes Weiner <hannes@cmpxchg.org>]

Hi David,

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