Ah, sig->notify_count = zap_other_threads(tsk) is the same variable.
Sorry for the noise.
Thanks,
Kirill
В Чт, 05/02/2015 в 19:11 +0300, Kirill Tkhai пишет:
> В Чт, 05/02/2015 в 14:38 +0100, Oleg Nesterov пишет:
> > 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().
> >
> > How?
> >
> > OK, yes, "sig->notify_count = -1" can be reordered with the last unlock,
> > but we do not care?
> >
> > group_exit_task + notify_count is only checked under the same lock, and
> > "notify_count = -1" can't happen until de_thread() sees it is zero.
> >
> > Could you explain why this is bad in more details?
> >
> >
> > > --- 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);
> >
> > Perhaps something like this makes sense anyway to make the code more
> > clear, but in this case I'd suggest to set ->notify_count after we
> > check ->exit_state. And without the (afaics!) misleading comment...
> >
> > Or I missed something?
>
> Other solution is in the patch below.
>
> Can't (sig->notify_count == -1) be visible earlier than
> tsk->signal->group_exit_task
> in exit_notify()?
>
> tasklist_lock is held in exit_notify(), but de_thread() actions (notify_count
> and
> group_exit_task writes) are independent from it (another lock is held there).
>
> diff --git a/fs/exec.c b/fs/exec.c
> index ad8798e..e3235b7 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -920,6 +920,7 @@ static int de_thread(struct task_struct *tsk)
> if (!thread_group_leader(tsk)) {
> struct task_struct *leader = tsk->group_leader;
>
> + smp_wmb(); /* Pairs with smp_rmb() in exit_notify */
> sig->notify_count = -1; /* for exit_notify() */
> for (;;) {
> threadgroup_change_begin(tsk);
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 6806c55..665fe0e 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -615,8 +615,10 @@ static void exit_notify(struct task_struct *tsk, int
> group_dead)
> list_add(&tsk->ptrace_entry, &dead);
>
> /* mt-exec, de_thread() is waiting for group leader */
> - if (unlikely(tsk->signal->notify_count < 0))
> + if (unlikely(tsk->signal->notify_count < 0)) {
> + smp_rmb(); /* Pairs with smp_wmb() in de_thread */
> wake_up_process(tsk->signal->group_exit_task);
> + }
> write_unlock_irq(&tasklist_lock);
>
> list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
>
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