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

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Date:	Tue, 6 Sep 2011 21:26:26 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Eric Dumazet <eric.dumazet@...il.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

On 09/06, Thomas Gleixner wrote:
>
> On Tue, 6 Sep 2011, Oleg Nesterov wrote:
> >
> > The problem is, it can be already dequeued.
>
> Right, but we can solve this by moving the whole detach code into rcu.

Hmm, I don't understand...

> --- 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);

Why do we need to move put_pid/sigqueue_free ?

The caller of release_posix_timer() should cancel the timer, we can can
do this even before idr_remove() with or without this patch.

>  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);

But how this can help? Suppose that the task is preempted right
after dequeue_signal() drops ->siglock. We need rcu_read_lock()
before unlock then, no?

And. This breaks the accounting logic. I mean the patch from Andi
which adds the limits.

Oleg.

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