On 08/05/2016 02:38 PM, Nathan Sidwell wrote: > On 08/05/16 04:55, Martin Liška wrote: > >> Thank you for very intensive brainstorming ;) Well I still believe that >> following code >> is not thread safe, let's image following situation: >> > > yeah, you're right. > >>> we could do better by using compare_exchange storing value, and detect the >>> race I mentioned: >>> >>> gcov_t expected, val; >>> atomic_load (&counter[0], &val, ...); >> >> [thread 1]: value == 1, read val == 1 // scheduled here >> >>> gcov_t delta = val == value ? 1 : -1; >>> atomic_add (&counter[1], delta); >>> if (delta < 0) { >>> retry: >>> /* can we set counter[0]? */ >>> atomic_load (&counter[1], &expected, ...); >>> if (expected < 0) { >>> bool stored = atomic_compare_exchange (&counter[0], &val, &value, >>> ...); >>> if (!stored && val != value) >>> goto retry; >> [thread 2]: value == 2, just updated counter[0] to 2 >> // after that [thread 1] continue, but wrongly does counter[1]++, but value >> != counter[0] >>> atomic_add (&counter[1], 2, ...); > > Bah. but (a) does it matter enough? and (b) if so does changing the delta<0 > handling to store a count of 1 solve it?: (answer: no) > > gcov_t expected, val; > A:atomic_load (&counter[0], &val, ...); > gcov_t delta = val == value ? 1 : -1; > B:atomic_add (&counter[1], delta); > > if (delta < 0) { > /* can we set counter[0]? */ > C:atomic_load (&counter[1], &expected, ...); > if (expected < 0) { > D:atomic_store (&counter[0], &value); > E: atomic_store (&counter[1], 1); > } > atomic_add (&counter[1], 2, ...); > > > thread-1: value = 1, reads '1' at A > thread-2: value = 2, reads '1' at A > thread-2: decrements count @ B > thread-2: reads -1 at C > thread-2: write 2 at D > thread-2: stores 1 at E > thread-1: increments count @ B (finally) > > So we still can go awry. But the code's simpler. Like you said, I don't > think it's possible to solve without an atomic update to both counter[0] & > counter[1]. > > >> Well, I wrote attached test-case which should trigger a data-race, but TSAN >> is silent: > > I'm not too surprised. The race window is tiny and you put a printf in the > middle of one path. I suspect if you put a sleep/printf on the counter[1] > increment path you'll see it more often. > > nathan
Ok, after all the experimenting and inventing "almost" thread-safe code, I incline to not to include __gcov_one_value_profiler_body_atomic counter. The final solution is cumbersome and probably does not worth doing. Moreover, even having a thread-safe implementation, result of an indirect call counter is not going to be stable among different runs (due to a single value storage capability). If you agree, I'll prepare a final version of patch? Thanks, Martin