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/ - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
