linux-kernel - Re: [PATCH] de_thread: Move notify

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Date:	Thu, 5 Feb 2015 19:49:38 +0300
From:	Kirill Tkhai <ktkhai@...allels.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	<linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, <tkhai@...dex.ru>
Subject: Re: [PATCH] de_thread: Move notify_count write under lock

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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