Re: [Valgrind-users] Helgrind detects race with same lock

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You might have been unlucky and have a lock that was freed and then
re-used.

See extract of mk_LockP_from_LockN comments:
   So we check that each LockN is a member of the admin_locks double
   linked list of all Lock structures.  That stops us prodding around
   in potentially freed-up Lock structures.  However, it's not quite a
   proper check: if a new Lock has been reallocated at the same
   address as one which was previously freed, we'll wind up copying
   the new one as the basis for the LockP, which is completely bogus
   because it is unrelated to the previous Lock that lived there.
   Let's hope that doesn't happen too often.

Do you have a small reproducer for the below ?
Philippe

On Mon, 2017-05-29 at 17:33 +0000, William Good wrote:
> Hello,
> 
> I am trying to understand this helgrind output.  It says there is a
> data-race on a read.  However both threads hold the same lock.  How
> can this be a race when both threads hold the lock during the access?
> 
> 
> ==31341==
> ----------------------------------------------------------------
> ==31341==
> ==31341==  Lock at 0x5990828 was first observed
> ==31341==    at 0x4C31A76: pthread_mutex_init (hg_intercepts.c:779)
> ==31341==    by 0x4026AF: thread_pool_submit (threadpool.c:85)
> ==31341==    by 0x402012: qsort_internal_parallel (quicksort.c:142)
> ==31341==    by 0x402040: qsort_internal_parallel (quicksort.c:151)
> ==31341==    by 0x402040: qsort_internal_parallel (quicksort.c:151)
> ==31341==    by 0x402450: thread_work (threadpool.c:233)
> ==31341==    by 0x4C3083E: mythread_wrapper (hg_intercepts.c:389)
> ==31341==    by 0x4E42DC4: start_thread
> (in /usr/lib64/libpthread-2.17.so)
> ==31341==    by 0x5355CEC: clone (in /usr/lib64/libc-2.17.so)
> ==31341==  Address 0x5990828 is 40 bytes inside a block of size 152
> alloc'd
> ==31341==    at 0x4C2CD95: calloc (vg_replace_malloc.c:711)
> ==31341==    by 0x4026A1: thread_pool_submit (threadpool.c:84)
> ==31341==    by 0x402012: qsort_internal_parallel (quicksort.c:142)
> ==31341==    by 0x402040: qsort_internal_parallel (quicksort.c:151)
> ==31341==    by 0x402040: qsort_internal_parallel (quicksort.c:151)
> ==31341==    by 0x40279F: future_get (threadpool.c:112)
> ==31341==    by 0x402048: qsort_internal_parallel (quicksort.c:152)
> ==31341==    by 0x402040: qsort_internal_parallel (quicksort.c:151)
> ==31341==    by 0x402450: thread_work (threadpool.c:233)
> ==31341==    by 0x4C3083E: mythread_wrapper (hg_intercepts.c:389)
> ==31341==    by 0x4E42DC4: start_thread
> (in /usr/lib64/libpthread-2.17.so)
> ==31341==    by 0x5355CEC: clone (in /usr/lib64/libc-2.17.so)
> ==31341==  Block was alloc'd by thread #3
> ==31341==
> ==31341== Possible data race during read of size 4 at 0x5990880 by
> thread #2
> ==31341== Locks held: 1, at address 0x5990828
> ==31341==    at 0x4023A9: thread_work (threadpool.c:229)
> ==31341==    by 0x4C3083E: mythread_wrapper (hg_intercepts.c:389)
> ==31341==    by 0x4E42DC4: start_thread
> (in /usr/lib64/libpthread-2.17.so)
> ==31341==    by 0x5355CEC: clone (in /usr/lib64/libc-2.17.so)
> ==31341==
> ==31341== This conflicts with a previous write of size 4 by thread #3
> ==31341== Locks held: 1, at address 0x5990828
> ==31341==    at 0x4027B3: future_get (threadpool.c:114)
> ==31341==    by 0x402048: qsort_internal_parallel (quicksort.c:152)
> ==31341==    by 0x402040: qsort_internal_parallel (quicksort.c:151)
> ==31341==    by 0x40279F: future_get (threadpool.c:112)
> ==31341==    by 0x402048: qsort_internal_parallel (quicksort.c:152)
> ==31341==    by 0x40279F: future_get (threadpool.c:112)
> ==31341==    by 0x402048: qsort_internal_parallel (quicksort.c:152)
> ==31341==    by 0x402040: qsort_internal_parallel (quicksort.c:151)
> ==31341==  Address 0x5990880 is 128 bytes inside a block of size 152
> alloc'd
> ==31341==    at 0x4C2CD95: calloc (vg_replace_malloc.c:711)
> ==31341==    by 0x4026A1: thread_pool_submit (threadpool.c:84)
> ==31341==    by 0x402012: qsort_internal_parallel (quicksort.c:142)
> ==31341==    by 0x402040: qsort_internal_parallel (quicksort.c:151)
> ==31341==    by 0x402040: qsort_internal_parallel (quicksort.c:151)
> ==31341==    by 0x40279F: future_get (threadpool.c:112)
> ==31341==    by 0x402048: qsort_internal_parallel (quicksort.c:152)
> ==31341==    by 0x402040: qsort_internal_parallel (quicksort.c:151)
> ==31341==    by 0x402450: thread_work (threadpool.c:233)
> ==31341==    by 0x4C3083E: mythread_wrapper (hg_intercepts.c:389)
> ==31341==    by 0x4E42DC4: start_thread
> (in /usr/lib64/libpthread-2.17.so)
> ==31341==    by 0x5355CEC: clone (in /usr/lib64/libc-2.17.so)
> ==31341==  Block was alloc'd by thread #3
> ==31341==
> ==31341==
> ----------------------------------------------------------------
> 
> ------------------------------------------------------------------------------
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