On Wed, Apr 01, 2020 at 08:34:36AM +0200, Marco Elver wrote: > On Tue, 31 Mar 2020 at 15:10, Will Deacon <will@xxxxxxxxxx> wrote: > > On Tue, Mar 24, 2020 at 05:23:30PM +0100, Marco Elver wrote: > > > Then, my suggestion would be to mark the write with data_race() and > > > just leave this as a READ_ONCE(). Having a data_race() somewhere only > > > makes KCSAN stop reporting the race if the paired access is also > > > marked (be it with data_race() or _ONCE, etc.). > > > > The problem with taking that approach is that it ends up much of the > > list implementation annotated with either WRITE_ONCE() or data_race(), > > meaning that concurrent, racy list operations will no longer be reported > > by KCSAN. I think that's a pretty big deal and I'm strongly against > > annotating the internals of library code such as this because it means > > that buggy callers will largely go undetected. > > > > The situation we have here is that some calls, e.g. hlist_empty() are > > safe even in the presence of a racy write and I'd like to suppress KCSAN > > reports without annotating the writes at all. > > > > > Alternatively, if marking the write is impossible, you can surround > > > the access with kcsan_disable_current()/kcsan_enable_current(). Or, as > > > a last resort, just leaving as-is is fine too, because KCSAN's default > > > config (still) has KCSAN_ASSUME_PLAIN_WRITES_ATOMIC selected. > > > > Hmm, I suppose some bright spark will want to change the default at the some > > point though, no? ;) I'll look at using > > kcsan_disable_current()/kcsan_enable_current(), thanks. > > I think this will come up again (it did already come up in some other > patch I reviewed, and Paul also mentioned it), so it seems best to > change data_race() to match the intuitive semantics of just completely > ignoring the access marked with it. I.e. marking accesses racing with > accesses marked with data_race() is now optional: > https://lkml.kernel.org/r/20200331193233.15180-1-elver@xxxxxxxxxx /me goes look. Thanks! > In which case, the original patch you had here works just fine. Ah yes, so now data_race(READ_ONCE(...)) does make sense as a combination. It's tempting to wrap that up as an accessor, but actually forcing people to spell it out might not be a bad thing after all. Will
