On Fri, 8 Nov 2019 at 19:40, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Fri, Nov 8, 2019 at 10:16 AM Marco Elver <elver@xxxxxxxxxx> wrote: > > > > KCSAN does not use volatile to distinguish accesses. Right now > > READ_ONCE, WRITE_ONCE, atomic bitops, atomic_t (+ some arch specific > > primitives) are treated as marked atomic operations. > > Ok, so we'd have to do this in terms of ATOMIC_WRITE(). > > One alternative might be KCSAN enhancement, where you notice the > following pattern: > > - a field is initialized before the data structure gets exposed (I > presume KCSAN already must understand about this issue - > initializations are different and not atomic) > > - while the field is live, there are operations that write the same > (let's call it "idempotent") value to the field under certain > circumstances > > - at release time, after all the reference counts are gone, the field > is read for whether that situation happened. I'm assuming KCSAN > already understands about this case too? It's not explicitly aware of initialization or release. We rely on compiler instrumentation for all memory accesses; KCSAN then sets up "watchpoints" for sampled memory accesses, delaying execution, and checking if a concurrent access is observed. We already have an option (currently disabled on syzbot) where KCSAN infers a data race not because another instrumented accesses happened concurrently, but because the data value changed during a watchpoint's lifetime (e.g. due to uninstrumented write, device write etc.). This same approach could be used to ignore "idempotent writes" where we would otherwise report a data race; i.e. if there was a concurrent write, but the data value did not change, do not report the race. I'm happy to add this feature if this should always be ignored. > So it would only be the "idempotent writes" thing that would be > something KCSAN would have to realize do not involve a race - because > it simply doesn't matter if two writes of the same value race against > each other. > > But I guess we could also just do > > #define WRITE_IDEMPOTENT(x,y) WRITE_ONCE(x,y) > > and use that in the kernel to annotate these things. And if we have > that kind of annotation, we could then possibly change it to > > #define WRITE_IDEMPOTENT(x,y) \ > if READ_ONCE(x)!=y WRITE_ONCE(x,y) > > if we have numbers that that actually helps (that macro written to be > intentionally invalid C - it obviously needs statement protection and > protection against evaluating the arguments multiple times etc). > > Linus Thanks, -- Marco
