linux-kernel - Re: Q: perf_install_in_context/perf_event

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Date:	Thu, 27 Jan 2011 17:59:24 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: perf_install_in_context/perf_event_enable are racy?

On 01/27, Peter Zijlstra wrote:
>
> On Thu, 2011-01-27 at 17:10 +0100, Oleg Nesterov wrote:
> > >  #ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
> > >       local_irq_enable();
> > > +     rq->in_ctxsw = 0;
> >
> > If we think that context_switch finishes here, probably it would be
> > more clean to clear ->in_ctxsw before local_irq_enable().
>
> It must in fact be done before,

Yes, I alredy realized this when I was reading another email from you.

> > But, otoh, maybe finish_lock_switch() can clear in_ctxsw, it already
> > checks __ARCH_WANT_INTERRUPTS_ON_CTXSW. Likewise, perhaps it can be
> > set in prepare_lock_switch() which enables irqs.
> >
> > But this is cosmetic and up to you.
>
> Can't do because of the above thing..

Right.

Oleg.

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