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

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Message-ID: <alpine.LFD.2.02.1109221317130.2723@ionos>
Date:	Thu, 22 Sep 2011 13:19:48 +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>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 4/4] posix-timers: turn it_signal into it_valid flag

On Wed, 21 Sep 2011, Thomas Gleixner wrote:
> On Wed, 21 Sep 2011, Thomas Gleixner wrote:
> > 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.
> > 
> > sys_private is ugly as hell and we should avoid to add another field
> > to siginfo.
> > 
> > I think we can embed the timer siginfo into k_itimer instead and
> 
> That should be sigqeue of course, which has siginfo embedded.

Bah, dequeue_signal only gets the copy of the siginfo. So we need to
do something about that in any case (*my_timer or embedded sigqueue).

Thanks,

	tglx
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