Herbert Xu wrote:

On Thu, Aug 16, 2007 at 04:56:21PM +1000, Paul Mackerras wrote:

quoted

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?

quoted

- 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.

quoted

- 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
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http://arcgraph.de/sr/