linux-kernel - Re: [RFC][PATCH 5/5] signals: Don't hold shared siglock across signal delivery

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Message-ID: <20110414192059.GB23517@redhat.com>
Date:	Thu, 14 Apr 2011 21:20:59 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Matt Fleming <matt.fleming@...ux.intel.com>
Subject: Re: [RFC][PATCH 5/5] signals: Don't hold shared siglock across
	signal delivery

On 04/14, Matt Fleming wrote:
>
> On Wed, 13 Apr 2011 22:12:19 +0200
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > This adds new races. And this time I do not even understand the intent.
> > I mean, it is not clear to me why this change can really help to speed
> > up get_signal_to_deliver().
>
> Again, it's not necessarily speeding up get_signal_to_deliver(), but
> rather it's reducing the contention on the shared siglock.

Yes, sorry for confusion. I used the "speed up" term wrongly throughout.
I understand what are you trying to do.

But yes, in this case I probably missed the intent,

> For example, without this patch, if you've got someone sending a signal
> to a task group, you can't run get_signal_to_deliver() in parallel

I missed the simple fact, get_signal_to_deliver() could avoid ->siglock
completely if it dequeues the private signal.

Btw, I forgot to mention another problem. We should not dequeue from
signal->shared_pending before task->pending. There are various reasons
why we shouldn't, but in particular please look at a27341cd
"Prioritize synchronous signals over 'normal' signals".

Oleg.

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