netdev - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Date:	Thu, 16 Aug 2007 09:22:44 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Satyam Sharma <satyam@...radead.org>
Cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	Christoph Lameter <clameter@....com>,
	Chris Snook <csnook@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	netdev@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	ak@...e.de, heiko.carstens@...ibm.com, davem@...emloft.net,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org,
	rpjday@...dspring.com, jesper.juhl@...il.com,
	segher@...nel.crashing.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures

Satyam Sharma writes:

> > Doesn't "atomic WRT all processors" require volatility?
> 
> No, it definitely doesn't. Why should it?
> 
> "Atomic w.r.t. all processors" is just your normal, simple "atomicity"
> for SMP systems (ensure that that object is modified / set / replaced
> in main memory atomically) and has nothing to do with "volatile"
> behaviour.

Atomic variables are "volatile" in the sense that they are liable to
be changed at any time by mechanisms that are outside the knowledge of
the C compiler, namely, other CPUs, or this CPU executing an interrupt
routine.

In the kernel we use atomic variables in precisely those situations
where a variable is potentially accessed concurrently by multiple
CPUs, and where each CPU needs to see updates done by other CPUs in a
timely fashion.  That is what they are for.  Therefore the compiler
must not cache values of atomic variables in registers; each
atomic_read must result in a load and each atomic_set must result in a
store.  Anything else will just lead to subtle bugs.

I have no strong opinion about whether or not the best way to achieve
this is through the use of the "volatile" C keyword.  Segher's idea of
using asm instead seems like a good one to me.

Paul.
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