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

[David Rientjes <rientjes@google.com>]

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.

I assume that's why Michal implemented it this way and added the comment 
to the lmk in commit 49550b605587 ("oom: add helpers for setting and 
clearing TIF_MEMDIE") to avoid TIF_MEMDIE entirely there.

> ---
>  mm/oom_kill.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index b2f081fe4b1a..4b9547be9170 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -435,8 +435,7 @@ void mark_oom_victim(struct task_struct *tsk)
>   */
>  void exit_oom_victim(void)
>  {
> -	if (!test_and_clear_thread_flag(TIF_MEMDIE))
> -		return;
> +	clear_thread_flag(TIF_MEMDIE);
>  
>  	down_read(&oom_sem);
>  	/*