http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55561
--- Comment #20 from Dmitry Vyukov <dvyukov at google dot com> 2012-12-29 10:13:00 UTC --- (In reply to comment #19) > On Wed, Dec 26, 2012 at 12:23 AM, Joost.VandeVondele at mat dot > ethz.ch <gcc-bugzi...@gcc.gnu.org> wrote: > > > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55561 > > > > --- Comment #16 from Joost VandeVondele <Joost.VandeVondele at mat dot > > ethz.ch> 2012-12-25 20:23:07 UTC --- > > many things appear to work fine, but seemingly parallel do loops with a > > dynamic > > schedule generate warnings in libgomp. I also seem to observe that they are > > not > > strictly deterministic, sometimes these warnings happen, sometimes not. > > > > Testcase: > > > > !$OMP PARALLEL PRIVATE(j) > > > > j=OMP_GET_THREAD_NUM() > > > > ! no warnings without the dynamic schedule > > !$OMP DO SCHEDULE(DYNAMIC,2) > > DO i=1,10 > > ENDDO > > > > !$OMP END PARALLEL > > END > > > > Result: > > > > vjo...@nanosim-s01.ethz.ch:/data/vjoost/clean/cp2k/cp2k/src> ./a.out > > vjo...@nanosim-s01.ethz.ch:/data/vjoost/clean/cp2k/cp2k/src> ./a.out > > vjo...@nanosim-s01.ethz.ch:/data/vjoost/clean/cp2k/cp2k/src> ./a.out > > vjo...@nanosim-s01.ethz.ch:/data/vjoost/clean/cp2k/cp2k/src> ./a.out > > ================== > > WARNING: ThreadSanitizer: data race (pid=35190) > > Read of size 8 at 0x7d3000027290 by main thread: > > #0 gomp_iter_dynamic_next > > /data/vjoost/gnu/gcc_trunk/obj/x86_64-unknown-linux-gnu/libgomp/../../../gcc/libgomp/iter.c:190 > > (libgomp.so.1+0x000000006678) > > #1 GOMP_loop_dynamic_start > > /data/vjoost/gnu/gcc_trunk/obj/x86_64-unknown-linux-gnu/libgomp/../../../gcc/libgomp/loop.c:128 > > (libgomp.so.1+0x000000007a03) > > #2 MAIN__._omp_fn.0 test.f90:0 (exe+0x000000000d7d) > > #3 MAIN__ test.f90:0 (exe+0x000000000ccb) > > #4 main ??:0 (exe+0x000000000d1a) > > > > Previous write of size 8 at 0x7d3000027290 by thread 1: > > #0 gomp_loop_init > > /data/vjoost/gnu/gcc_trunk/obj/x86_64-unknown-linux-gnu/libgomp/../../../gcc/libgomp/loop.c:41 > > (libgomp.so.1+0x000000007a96) > > #1 MAIN__._omp_fn.0 test.f90:0 (exe+0x000000000d7d) > > #2 gomp_thread_start > > /data/vjoost/gnu/gcc_trunk/obj/x86_64-unknown-linux-gnu/libgomp/../../../gcc/libgomp/team.c:116 > > (libgomp.so.1+0x00000000d012) > > > > Location is heap block of size 1568 at 0x7d3000027100 allocated by main > > thread: > > #0 malloc ??:0 (libtsan.so.0+0x00000001896e) > > #1 gomp_malloc > > /data/vjoost/gnu/gcc_trunk/obj/x86_64-unknown-linux-gnu/libgomp/../../../gcc/libgomp/alloc.c:36 > > (libgomp.so.1+0x00000000417a) > > #2 gomp_new_team > > /data/vjoost/gnu/gcc_trunk/obj/x86_64-unknown-linux-gnu/libgomp/../../../gcc/libgomp/team.c:145 > > (libgomp.so.1+0x00000000d27a) > > #3 GOMP_parallel_start > > /data/vjoost/gnu/gcc_trunk/obj/x86_64-unknown-linux-gnu/libgomp/../../../gcc/libgomp/parallel.c:108 > > (libgomp.so.1+0x00000000afc7) > > #4 MAIN__ test.f90:0 (exe+0x000000000cc1) > > #5 main ??:0 (exe+0x000000000d1a) > > > > Thread 1 (tid=35191, running) created at: > > #0 pthread_create ??:0 (libtsan.so.0+0x00000001a868) > > #1 gomp_team_start > > /data/vjoost/gnu/gcc_trunk/obj/x86_64-unknown-linux-gnu/libgomp/../../../gcc/libgomp/team.c:440 > > (libgomp.so.1+0x00000000d908) > > #2 GOMP_parallel_start > > /data/vjoost/gnu/gcc_trunk/obj/x86_64-unknown-linux-gnu/libgomp/../../../gcc/libgomp/parallel.c:108 > > (libgomp.so.1+0x00000000afd7) > > #3 MAIN__ test.f90:0 (exe+0x000000000cc1) > > #4 main ??:0 (exe+0x000000000d1a) > > > Looks like unsafe publication of gomp_work_share data. > > Can you show disassembly of > > #2 MAIN__._omp_fn.0 test.f90:0 (exe+0x000000000d7d) > ? > How does it choose between calling gomp_loop_init() and > GOMP_loop_dynamic_start()? > > Humm... do omp generated functions (like MAIN__._omp_fn.0) pass > through tsan pass? Perhaps it contains some atomic op that tsan does > not see. Congratulations! We've found racy unsafe publication in libgomp with ThreadSanitizer: gomp_loop_dynamic_start() uses the following functions to synchronize with each other. As you can see gomp_ptrlock_get() contains fast-and-dead unsafe fast-path. libgomp/config/posix/ptrlock.h static inline void *gomp_ptrlock_get (gomp_ptrlock_t *ptrlock) { if (ptrlock->ptr != NULL) return ptrlock->ptr; gomp_mutex_lock (&ptrlock->lock); if (ptrlock->ptr != NULL) { gomp_mutex_unlock (&ptrlock->lock); return ptrlock->ptr; } return NULL; } static inline void gomp_ptrlock_set (gomp_ptrlock_t *ptrlock, void *ptr) { ptrlock->ptr = ptr; gomp_mutex_unlock (&ptrlock->lock); }
