linux-kernel - Re: [PATCH 4/4] posix-timers: turn it_signal into it

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Message-ID: <20110906162718.GA19030@redhat.com>
Date:	Tue, 6 Sep 2011 18:27:18 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/4] posix-timers: turn it_signal into it_valid flag

On 09/06, Eric Dumazet wrote:
>
> Le mardi 06 septembre 2011 à 16:51 +0200, Oleg Nesterov a écrit :
> > On 09/05, Andi Kleen wrote:
> > >
> > > > I forgot everything I knew about ->it_requeue_pending logic, but it
> > > > seems to me that do_schedule_next_timer()->lock_timer() can find and
> > > > lock successfully the wrong timer. Another thread can do timer_delete()
> > > > and then re-create the timer with the same id.
> > >
> > > Do you mean after my patches or even before?
> >
> > Ah, sorry for confusion.
> >
> > Before. And after. IOW, I think this has nothing to do with your patches.
> >
>
> Hmm, you mean following patch is needed ?
> 
> Before release of timer id to idr pool, we should make sure
> do_schedule_next_timer() wont be called, or it could find another timer
> reusing the just released id.
>
> diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
> index 4556182..4369747 100644
> --- a/kernel/posix-timers.c
> +++ b/kernel/posix-timers.c
> @@ -502,14 +502,14 @@ static void k_itimer_rcu_free(struct rcu_head *head)
>  #define IT_ID_NOT_SET	0
>  static void release_posix_timer(struct k_itimer *tmr, int it_id_set)
>  {
> +	put_pid(tmr->it_pid);
> +	sigqueue_free(tmr->sigq);
>  	if (it_id_set) {
>  		unsigned long flags;
>  		spin_lock_irqsave(&idr_lock, flags);
>  		idr_remove(&posix_timers_id, tmr->it_id);
>  		spin_unlock_irqrestore(&idr_lock, flags);
>  	}
> -	put_pid(tmr->it_pid);
> -	sigqueue_free(tmr->sigq);

I don't think this can make any difference. We simply can't guarantee
do_schedule_next_timer() won't be called.

We could mark tmr->sigq as "invalid", but even this can't help. Suppose
that the task has already dequeued the __SI_TIMER signal, now it plays
withe the copy of tmr->sigq.

Oleg.

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