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-bugzi...@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-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);

> }

>

> --

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