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

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Date:	Wed, 15 Aug 2007 15:08:43 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Satyam Sharma <satyam@...radead.org>
CC:	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 wrote:
> On Wed, 15 Aug 2007, Stefan Richter wrote:
>> 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.
> 
> "Volatile behaviour" itself isn't consistently defined (at least
> definitely not consistently implemented in various gcc versions across
> platforms), but it is /expected/ to mean something like: "ensure that
> every such access actually goes all the way to memory, and is not
> re-ordered w.r.t. to other accesses, as far as the compiler can take
> care of these". The last "as far as compiler can take care" disclaimer
> comes about due to CPUs doing their own re-ordering nowadays.
> 
> For example (say on i386):

[...]

> In (A) the compiler optimized "a = 10;" away, but the actual store
> of the final value "20" to "a" was still "atomic". (B) and (C) also
> exhibit "volatile" behaviour apart from the "atomicity".
> 
> But as others replied, it seems some callers out there depend upon
> atomic ops exhibiting "volatile" behaviour as well, so that answers
> my initial question, actually. I haven't looked at the code Paul
> pointed me at, but I wonder if that "forget(x)" macro would help
> those cases. I'd wish to avoid the "volatile" primitive, personally.

So, looking at load instead of store, understand I correctly that in
your opinion

	int b;

	b = atomic_read(&a);
	if (b)
		do_something_time_consuming();

	b = atomic_read(&a);
	if (b)
		do_something_more();

should be changed to explicitly forget(&a) after
do_something_time_consuming?

If so, how about the following:

static inline void A(atomic_t *a)
{
	int b = atomic_read(a);
	if (b)
		do_something_time_consuming();
}

static inline void B(atomic_t *a)
{
	int b = atomic_read(a);
	if (b)
		do_something_more();
}

static void C(atomic_t *a)
{
	A(a);
	B(b);
}

Would this need forget(a) after A(a)?

(Is the latter actually answered in C99 or is it compiler-dependent?)
-- 
Stefan Richter
-=====-=-=== =--- -====
http://arcgraph.de/sr/
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