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

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Message-Id: <1238481425.28248.1377.camel@twins>
Date:	Tue, 31 Mar 2009 08:37:05 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/15] perf_counter: unify and fix delayed counter
 wakeup

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 :-)

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