[Bug sanitizer/55561] TSAN: Fortran/OMP yields false positives

["dvyukov at google dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55561

--- Comment #21 from Dmitry Vyukov <dvyukov at google dot com> 2012-12-29 10:21:06 UTC ---
It is a known issue http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40362#c7
and Jakub said it's safe. I am not that sure.

On Sat, Dec 29, 2012 at 2:13 PM, dvyukov at google dot com
<gcc-bugzilla@gcc.gnu.org> wrote:
>
> 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-bugzilla@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:
>> >
>> > vjoost@nanosim-s01.ethz.ch:/data/vjoost/clean/cp2k/cp2k/src> ./a.out
>> > vjoost@nanosim-s01.ethz.ch:/data/vjoost/clean/cp2k/cp2k/src> ./a.out
>> > vjoost@nanosim-s01.ethz.ch:/data/vjoost/clean/cp2k/cp2k/src> ./a.out
>> > vjoost@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);
> }
>
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