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

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Date:	Tue, 06 Sep 2011 21:30:11 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Oleg Nesterov <oleg@...hat.com>, Andi Kleen <ak@...ux.intel.com>,
	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

Le mardi 06 septembre 2011 à 21:16 +0200, Thomas Gleixner a écrit :
> On Tue, 6 Sep 2011, Oleg Nesterov wrote:
> > On 09/06, Thomas Gleixner wrote:
> > >
> > > On Tue, 6 Sep 2011, 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.
> > >
> > > I don't see how that makes it sure. If the signal is queued, then it
> > > stays queued and the put_pid() has no effect either.
> > 
> > Yes.
> > 
> > But "If the signal is queued" is simple, in this case we could, say,
> > do "tmr->sigq->info.si_tid = -1" to ensure lock_timer()->find_idr()
> > can't succeed after dequeue_signal().
> > 
> > The problem is, it can be already dequeued.
> 
> Right, but we can solve this by moving the whole detach code into rcu.
> 

Why ? Is the dequeue thing guaranteed in the rcu grace period ?

ALso, delaying the idr_remove() probably makes next Andi patch more
complex (move global timer id management to signal_struct)

struct k_itimer will need a backpointer to signal_struct, and an
additional refcount on it.


> Subject: posix-timer-fix-detach-race.patch
> From: Thomas Gleixner <tglx@...utronix.de>
> Date: Tue, 06 Sep 2011 21:08:06 +0200
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/posix-timers.c |   26 +++++++++++++++++---------
>  1 file changed, 17 insertions(+), 9 deletions(-)
> 
> Index: linux-2.6/kernel/posix-timers.c
> ===================================================================
> --- linux-2.6.orig/kernel/posix-timers.c
> +++ linux-2.6/kernel/posix-timers.c
> @@ -495,22 +495,30 @@ static void k_itimer_rcu_free(struct rcu
>  {
>  	struct k_itimer *tmr = container_of(head, struct k_itimer, it.rcu);
>  
> +	put_pid(tmr->it_pid);
> +	sigqueue_free(tmr->sigq);
>  	kmem_cache_free(posix_timers_cache, tmr);
>  }
>  
> +static void k_itimer_rcu_free_idr(struct rcu_head *head)
> +{
> +	struct k_itimer *tmr = container_of(head, struct k_itimer, it.rcu);
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&idr_lock, flags);
> +	idr_remove(&posix_timers_id, tmr->it_id);
> +	spin_unlock_irqrestore(&idr_lock, flags);
> +	k_itimer_rcu_free(head);
> +}
> +
>  #define IT_ID_SET	1
>  #define IT_ID_NOT_SET	0
>  static void release_posix_timer(struct k_itimer *tmr, int it_id_set)
>  {
> -	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);
> -	call_rcu(&tmr->it.rcu, k_itimer_rcu_free);
> +	if (it_id_set)
> +		call_rcu(&tmr->it.rcu, k_itimer_rcu_free_idr);
> +	else
> +		call_rcu(&tmr->it.rcu, k_itimer_rcu_free);
>  }
>  
>  static struct k_clock *clockid_to_kclock(const clockid_t id)


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