linux-kernel - Re: [PATCH 1/23] document preferred use of volatile with atomic

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Message-ID: <46C239F0.3030004@redhat.com>
Date:	Tue, 14 Aug 2007 19:25:36 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Christoph Lameter <clameter@....com>
CC:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	Segher Boessenkool <segher@...nel.crashing.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	Chris Friesen <cfriesen@...tel.com>,
	"Robert P. J. Day" <rpjday@...dspring.com>
Subject: Re: [PATCH 1/23] document preferred use of volatile with atomic_t

Christoph Lameter wrote:
> On Tue, 14 Aug 2007, Chris Snook wrote:
> 
>>> volatile means that there is some vague notion of "read it now". But that
>>> really does not exist. Instead we control visibility via barriers (smp_wmb,
>>> smp_rmb). Would it not be best to not have volatile at all in atomic
>>> operations and let the barriers do the work?
>> From my reply in the other thread...
>>
>> But barriers force a flush of *everything* in scope, which we generally don't
>> want.  On the other hand, we pretty much always want to flush atomic_*
>> operations.  One way or another, we should be restricting the volatile
>> behavior to the thing that needs it.  On most architectures, this patch set
>> just moves that from the declaration, where it is considered harmful, to the
>> use, where it is considered an occasional necessary evil.
>>
>> If you really, *really* distrust the compiler that much, you shouldn't be
>> using barrier, since that uses volatile under the hood too.  You should just
>> go ahead and implement the atomic operations in assembler, like Segher
>> Boessenkool did for powerpc in response to my previous patchset.
> 
> From my reply on the other thread:
> 
> Maybe we need two read functions? One volatile, one not?

If we're going to do this, and I don't think we need to, I'd prefer that 
atomic_read() be volatile, and something like atomic_read_opt() be non-volatile, 
to discourage premature optimization.

> The atomic_read()s that I have in slub really do not care about when the 
> variables are read. And if volatile creates overhead then I rather not have it.

A single volatile access is no more expensive than a non-volatile access.  It's 
when you have dependencies that you start to see overhead.  If you're doing a 
bunch of atomic operations on the same atomic_t in quick succession, then you 
will see some overhead.  Of course, if you're doing that, I think you have a 
design problem.

On modern, register-rich CPUs with cache latencies of a couple clock cycles, 
volatile generally isn't as much of a performance hit as it used to be.  I think 
that going out of your way to avoid it would be premature optimization on modern 
hardware.

	-- Chris
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