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

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Message-ID: <alpine.LFD.2.02.1109070032380.2723@ionos>
Date:	Wed, 7 Sep 2011 00:34:09 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Oleg Nesterov <oleg@...hat.com>
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 Wed, 7 Sep 2011, Oleg Nesterov wrote:

> 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?

Sounds reasonable, though I have the same sleep deprivation problem as
you :) Will have a closer look when awake.

Thanks,

	tglx
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