On Sat, Oct 5, 2019 at 6:16 AM Dmitry Vyukov <dvyu...@google.com> wrote: > > On Sat, Oct 5, 2019 at 2:58 AM Eric Dumazet <eric.duma...@gmail.com> wrote: > > > This one is tricky. What I think we need to avoid is an onslaught of > > > patches adding READ_ONCE/WRITE_ONCE without a concrete analysis of the > > > code being modified. My worry is that Joe Developer is eager to get their > > > first patch into the kernel, so runs this tool and starts spamming > > > maintainers with these things to the point that they start ignoring KCSAN > > > reports altogether because of the time they take up. > > > > > > I suppose one thing we could do is to require each new > > > READ_ONCE/WRITE_ONCE > > > to have a comment describing the racy access, a bit like we do for memory > > > barriers. Another possibility would be to use atomic_t more widely if > > > there is genuine concurrency involved. > > > > > > > About READ_ONCE() and WRITE_ONCE(), we will probably need > > > > ADD_ONCE(var, value) for arches that can implement the RMW in a single > > instruction. > > > > WRITE_ONCE(var, var + value) does not look pretty, and increases register > > pressure. > > FWIW modern compilers can handle this if we tell them what we are trying to > do: > > void foo(int *p, int x) > { > x += __atomic_load_n(p, __ATOMIC_RELAXED); > __atomic_store_n(p, x, __ATOMIC_RELAXED); > } > > $ clang test.c -c -O2 && objdump -d test.o > > 0000000000000000 <foo>: > 0: 01 37 add %esi,(%rdi) > 2: c3 retq > > We can have syntactic sugar on top of this of course.
An interesting precedent come up in another KCSAN bug report. Namely, it may be reasonable for a compiler to use different optimization heuristics for concurrent and non-concurrent code. Consider there are some legal code transformations, but it's unclear if they are profitable or not. It may be the case that for non-concurrent code the expectation is that it's a profitable transformation, but for concurrent code it is not. So that may be another reason to communicate to compiler what we want to do, rather than trying to trick and play against each other. I've added the concrete example here: https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE#it-may-improve-performance