linux-kernel - Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats

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Date:	Fri, 22 Feb 2013 15:04:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats

On Fri, 2013-02-22 at 14:54 +0100, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Fri, 2013-02-22 at 13:50 +0100, Frederic Weisbecker wrote:
> 
> > > atomic64_read() and atomic64_set() are supposed to take care 
> > > of that, without even the need for _inc() or _add() parts 
> > > that use LOCK.
> > 
> > Are you sure? Generally atomic*_set() is not actually an 
> > atomic operation.
> 
> as per Documentation/atomic_ops.h:

I think the interesting part is:

"The setting is atomic in that the return values of the atomic
operations by
all threads are guaranteed to be correct reflecting either the value
that has
been set with this operation or set with another operation.  A proper
implicit
or explicit memory barrier is needed before the value set with the
operation
is guaranteed to be readable with atomic_read from another thread."

Which does give us the wanted guarantee, however:

I checked arch/x86/include/asm/atomic64_32.h and we use cmpxchg8b for
everything from _set() to _read(), which translates into 'horridly
stupendifyingly slow' for a number of machines, but coherent.

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