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Message-ID: <alpine.LFD.0.999.0708171420060.3666@enigma.security.iitk.ac.in>
Date: Fri, 17 Aug 2007 14:28:42 +0530 (IST)
From: Satyam Sharma <satyam@...radead.org>
To: Nick Piggin <nickpiggin@...oo.com.au>
cc: Stefan Richter <stefanr@...6.in-berlin.de>,
paulmck@...ux.vnet.ibm.com,
Herbert Xu <herbert@...dor.apana.org.au>,
Paul Mackerras <paulus@...ba.org>,
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
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
architectures
On Fri, 17 Aug 2007, Nick Piggin wrote:
> Stefan Richter wrote:
> [...]
> Just use spinlocks if you're not absolutely clear about potential
> races and memory ordering issues -- they're pretty cheap and simple.
I fully agree with this. As Paul Mackerras mentioned elsewhere,
a lot of authors sprinkle atomic_t in code thinking they're somehow
done with *locking*. This is sad, and I wonder if it's time for a
Documentation/atomic-considered-dodgy.txt kind of document :-)
> > Sure, now
> > that I learned of these properties I can start to audit code and insert
> > barriers where I believe they are needed, but this simply means that
> > almost all occurrences of atomic_read will get barriers (unless there
> > already are implicit but more or less obvious barriers like msleep).
>
> You might find that these places that appear to need barriers are
> buggy for other reasons anyway. Can you point to some in-tree code
> we can have a look at?
Such code was mentioned elsewhere (query nodemgr_host_thread in cscope)
that managed to escape the requirement for a barrier only because of
some completely un-obvious compilation-unit-scope thing. But I find such
an non-explicit barrier quite bad taste. Stefan, do consider plunking an
explicit call to barrier() there.
Satyam
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