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

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Date:	Wed, 7 Sep 2011 00:08:27 +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:
>
> > 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?
>
> Crap, you are right, but that's fortunately an easy to solve one :)

Yes, this is solvable. But I think we can do something better.

> > And. This breaks the accounting logic. I mean the patch from Andi
> > which adds the limits.
>
> That's a different problem and really, it does not break it by any
> means. When the timer is released, then the count is decreased and we
> can safely assume that the memory is going to be freed in the next
> grace period.

Yes, but this means we need the counter which we do not have.

I think we can avoid this problems. Although I am not sure, I am
already sleeping.

	- we add rcu_read_lock() into dequeueu_signal().

	- we add the new "struct k_itimer *my_timer" member into
	 siginfo._timer. Like _sys_private it is not passed to
	 user, and perhaps we can kill _sys_private later.

	 It is initialized in sys_timer_create() along with
	 info.si_tid/etc

	- release_posix_timer() nullifies tmr->sigq->my_timer

	- do_schedule_next_timer() does

	 	timr = info->my_timer;
	 	if (!timr)
	 		return;

		// protected by rcu

	 	spin_lock_irq(timr->it_lock);
	 	if (!timr->it_signal) {
	 		spin_unlock_irq();
	 		return;
	 	}

	 	....

This also avoids idr_find(), and we do not need to delay idr_remove().

Possible?

Oleg.

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