linux-kernel - Re: [PATCH 02/10] perf, trace: Use per-tracepoint-per-cpu hlist to track events

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Message-ID: <20100521103816.GG30108@nowhere>
Date:	Fri, 21 May 2010 12:38:19 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/10] perf, trace: Use per-tracepoint-per-cpu hlist to
	track events

On Fri, May 21, 2010 at 12:34:03PM +0200, Peter Zijlstra wrote:
> On Fri, 2010-05-21 at 12:21 +0200, Frederic Weisbecker wrote:
> > On Fri, May 21, 2010 at 12:19:09PM +0200, Peter Zijlstra wrote:
> > > On Fri, 2010-05-21 at 12:13 +0200, Frederic Weisbecker wrote:
> > > > > I assumed that after probe unregister a tracepoint callback doesn't
> > > > > happen, which then guarantees we should never get !head.
> > > 
> > > > I'm not sure about this. The tracepoints are called under rcu_read_lock(),
> > > > but there is not synchronize_rcu() after we unregister a tracepoint, which
> > > > means you can have a pending preempted one somewhere.
> > > > 
> > > > There is a call_rcu that removes the callbacks, but that only protect
> > > > the callback themselves. 
> > > 
> > > Ah, ok, so we should do probe_unregister + synchronize_sched().
> > > That should ensure __DO_TRACE() doesn't call into it anymore.
> > > 
> > > /me goes make a patch
> > > 
> > 
> > 
> > Yep. But that also means we need to rcu_dereference_sched() to access
> > the per cpu list of events.
> 
> Why?
> 
> The per-cpu vars are allocated and freed in a fully serialized manner,
> there should be no races what so ever.


Ah right, I was confused.

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