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Message-ID: <46C5672E.4060003@cyberone.com.au>
Date: Fri, 17 Aug 2007 19:15:26 +1000
From: Nick Piggin <piggin@...erone.com.au>
To: Satyam Sharma <satyam@...radead.org>
CC: Stefan Richter <stefanr@...6.in-berlin.de>,
paulmck@...ux.vnet.ibm.com,
Herbert Xu <herbert@...dor.apana.org.au>,
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Christoph Lameter <clameter@....com>,
Chris Snook <csnook@...hat.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
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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,
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Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
architectures
Satyam Sharma wrote:
>
> On Fri, 17 Aug 2007, Nick Piggin wrote:
>>>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.
It is very obvious. msleep calls schedule() (ie. sleeps), which is
always a barrier.
The "unobvious" thing is that you wanted to know how the compiler knows
a function is a barrier -- answer is that if it does not *know* it is not
a barrier, it must assume it is a barrier. If the whole msleep call chain
including the scheduler were defined static in the current compilation
unit, then it would still be a barrier because it would actually be able
to see the barriers in schedule(void), if nothing else.
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