Re: [PATCH] de_thread: Move notify_count write under lock

From: Oleg Nesterov
Date: Mon Feb 23 2015 - 15:20:05 EST


I still think that the changelog and the comment look a bit confusing...
it could simply say that exit_notify() can see these STORE's out of order.
And we can set ->notify_count after ->exit_state check, but again this is
cosmetic, I won't insist. The main problem with this patch is that it was
ignored ;)

Kirill, could you resend? Feel free to add my ack.

On 02/05, Kirill Tkhai wrote:
>
> The write operation may be reordered with the setting of group_exit_task.
> If so, this fires in exit_notify().
>
> Looks like, it's not good to add smp barriers for this case, especially
> in exit_notify(), so let's put the notify_count write under write lock.
>
> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> ---
> fs/exec.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index ad8798e..42782d5 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -920,10 +920,16 @@ static int de_thread(struct task_struct *tsk)
> if (!thread_group_leader(tsk)) {
> struct task_struct *leader = tsk->group_leader;
>
> - sig->notify_count = -1; /* for exit_notify() */
> for (;;) {
> threadgroup_change_begin(tsk);
> write_lock_irq(&tasklist_lock);
> + /*
> + * We could set it once outside the for() cycle, but
> + * this requires to use SMP barriers there and in
> + * exit_notify(), because the write operation may
> + * be reordered with the setting of group_exit_task.
> + */
> + sig->notify_count = -1; /* for exit_notify() */
> if (likely(leader->exit_state))
> break;
> __set_current_state(TASK_KILLABLE);
>
>
>

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