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

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Message-ID: <20070815162722.GD9645@linux.vnet.ibm.com>
Date:	Wed, 15 Aug 2007 09:27:22 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Satyam Sharma <satyam@...radead.org>,
	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, 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.

						Thanx, Paul
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