fix(sync): close data race on catchup() count read (F-32, #112)#160
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fix(sync): close data race on catchup() count read (F-32, #112)#160adequatelimited merged 1 commit intoaudit-fixesfrom
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The while(count && ecode == VEOK) condition at sync.c:138 reads the shared count outside any OMP_CRITICAL_ section, while the two decrements are inside critical sections. Per the OpenMP memory model, the flush on critical-exit is not paired with an unsynchronized reader, so the read can observe stale cached values (or be a data race by C11; TSan will flag it). Shadow the parameter with a local `volatile word32 count = count_in`. volatile forces a memory read each iteration; combined with the writers' implicit flush on critical-exit, this establishes the needed happens-before. Same treatment applied to the analogous ecode read in tfile.c under F-10. Analyzed the underflow claim from the original issue and confirmed it is not reachable -- each thread's loop body decrements at most once before breaking. See github.com//issues/112 comment for the trace.
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Summary
Closes #112 (F-32) — partial. The underflow concern in the original report was analyzed and determined not to be a real risk; see this comment on the issue for the per-thread decrement trace.
The real issue is the data race on the shared `count` read at sync.c:136:
Per the OpenMP memory model, the flush on critical-exit is not paired with an unsynchronized reader, so the read can observe stale cached values (and TSan will flag it as a race).
Marking the parameter `volatile` forces a memory read each iteration. Combined with the writers' implicit flush on critical-exit, this establishes the needed happens-before. Same treatment applied to the analogous `ecode` read in `tfile.c` under F-10.
Test plan