linux-kernel - Re: [PATCH 01/15] perf_counter: unify and fix delayed counter wakeup

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Message-ID: <20090401090433.GA27865@elte.hu>
Date:	Wed, 1 Apr 2009 11:04:33 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/15] perf_counter: unify and fix delayed counter
	wakeup


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Tue, 2009-03-31 at 16:45 +1100, Paul Mackerras wrote:
> > Peter Zijlstra writes:
> > 
> > > +void perf_counter_wakeup(struct perf_counter *counter)
> > > +{
> > > +	struct perf_mmap_data *data;
> > > +
> > > +	rcu_read_lock();
> > > +	data = rcu_dereference(counter->data);
> > > +	if (data) {
> > > +		(void)atomic_xchg(&data->wakeup, POLL_IN);
> > 
> > Really just a nit, but how is this atomic_xchg any different from
> > atomic_set(&data->wakeup, POLL_IN) aside from being slower?
> 
> Probably, I got my head in a twist, atomic_set() is simply an 
> unlocked assignment (although volatile), and I read the value 
> using a locked xchg().
> 
> I wasn't sure how these two would interact and so I chickened out 
> :-)

The fact that you needed to use an ugly cast to silence the compiler 
should have told you that we kernel developers never chicken out! :)

(And if we do, it's called an orderly tactical retreat.)

	Ingo
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