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

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Date:	Thu, 16 Aug 2007 10:06:31 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	Paul Mackerras <paulus@...ba.org>,
	Satyam Sharma <satyam@...radead.org>,
	Christoph Lameter <clameter@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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

Herbert Xu wrote:
> On Thu, Aug 16, 2007 at 04:56:21PM +1000, Paul Mackerras wrote:
>>
>> Note that I said these are the cases _where one might want to allow
>> caching_, so of course adding volatile doesn't help _these_ cases.
>> There are of course other cases where one definitely doesn't want to
>> allow the compiler to cache the value, such as when polling an atomic
>> variable waiting for another CPU to change it, and from my inspection
>> so far these cases seem to be the majority.
> 
> We've been through that already.  If it's a busy-wait it
> should use cpu_relax.  If it's scheduling away that already
> forces the compiler to reread anyway.
> 
> Do you have an actual example where volatile is needed?
> 
>> - It matches the normal expectation based on the name "atomic_read"
>> - It matches the behaviour of the other atomic_* primitives
> 
> Can't argue since you left out what those expectations
> or properties are.

We use atomic_t for data that is concurrently locklessly written and
read at arbitrary times.  My naive expectation as driver author (driver
maintainer) is that all atomic_t accessors, including atomic_read, (and
atomic bitops) work with the then current value of the atomic data.

>> - It avoids bugs in the cases where "volatile" behaviour is required
> 
> Do you (or anyone else for that matter) have an example of this?

The only code I somewhat know, the ieee1394 subsystem, was perhaps
authored and is currently maintained with the expectation that each
occurrence of atomic_read actually results in a load operation, i.e. is
not optimized away.  This means all atomic_t (bus generation, packet and
buffer refcounts, and some other state variables)* and likewise all
atomic bitops in that subsystem.

If that assumption is wrong, then what is the API or language primitive
to force a load operation to occur?


*)  Interesting what a quick LXR session in search for all atomic_t
usages in 'my' subsystem brings to light.  I now noticed an apparently
unused struct member in the bitrotting pcilynx driver, and more
importantly, a pairing of two atomic_t variables in raw1394 that should
be audited for race conditions and for possible replacement by plain int.
-- 
Stefan Richter
-=====-=-=== =--- =----
http://arcgraph.de/sr/
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