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

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Date:	Wed, 15 Aug 2007 22:43:37 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Chris Snook <csnook@...hat.com>, clameter@....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, 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
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures



On Wed, 15 Aug 2007, Paul E. McKenney wrote:

> On Wed, Aug 15, 2007 at 06:09:35PM +0200, Stefan Richter wrote:
> > Herbert Xu wrote:
> > > On Wed, Aug 15, 2007 at 08:05:38PM +0530, Satyam Sharma wrote:
> > >>> I don't know if this here is affected:
> > 
> > [...something like]
> > 	b = atomic_read(a);
> > 	for (i = 0; i < 4; i++) {
> > 		msleep_interruptible(63);
> > 		c = atomic_read(a);
> > 		if (c != b) {
> > 			b = c;
> > 			i = 0;
> > 		}
> > 	}
> > 
> > > Nope, we're calling schedule which is a rather heavy-weight
> > > barrier.
> > 
> > How does the compiler know that msleep() has got barrier()s?
> 
> Because msleep_interruptible() is in a separate compilation unit,
> the compiler has to assume that it might modify any arbitrary global.
> In many cases, the compiler also has to assume that msleep_interruptible()
> might call back into a function in the current compilation unit, thus
> possibly modifying global static variables.

Yup, I've just verified this with a testcase. So a call to any function
outside of the current compilation unit acts as a compiler barrier. Cool.
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