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

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Message-ID: <18115.59365.268987.406001@cargo.ozlabs.ibm.com>
Date:	Thu, 16 Aug 2007 16:00:05 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Christoph Lameter <clameter@....com>,
	Satyam Sharma <satyam@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	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
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

Herbert Xu writes:

> It doesn't matter.  The memory pressure flag is an *advisory*
> flag.  If we get it wrong the worst that'll happen is that we'd
> waste some time doing work that'll be thrown away.

Ah, so it's the "racy but I don't care because it's only an
optimization" case.  That's fine.  Somehow I find it hard to believe
that all the racy uses of atomic_read in the kernel are like that,
though. :)

> In any case, this actually illustrates why the addition of
> volatile is completely pointless.  Even if this code was
> broken, which it definitely is not, having the volatile
> there wouldn't have helped at all.

Yes, adding volatile to racy code doesn't somehow make it race-free.
Neither does using atomic_t, despite what some seem to believe.

I have actually started going through all the uses of atomic_read in
the kernel.  So far out of the first 100 I have found none where we
have two atomic_reads of the same variable and the compiler could
usefully use the value from the first as the result of the second.
But there's still > 2500 to go...

Paul.
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