[PATCH] libstdc++: testsuite: Address random failure in pthread_create() [PR54185]

[Lewis Hyatt]

On Tue, Aug 18, 2020 at 09:43:31AM +0100, Jonathan Wakely wrote:
> On 13/08/20 18:15 -0400, Lewis Hyatt via Libstdc++ wrote:
> > Hello-
> > 
> > The attached patch was discussed briefly on PR 54185 here:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54185#c14
> > The test case for this PR sometimes fails due to random failures in
> > pthread_create() that are not related to the original PR. This patch fixes
> > it up by ignoring those failures. The test case was designed to repeat the
> > same test 1000 times to attempt to reproduce a race condition, so I think is
> > OK if some of those iterations are simply skipped.
> > 
> > Thanks for taking a look at it; I can commit it if it makes sense.
> > 
> > -Lewis
> 
> > libstdc++: testsuite: Address random failure in pthread_create() [PR54185]
> > 
> > The test for this PR calls pthread_create() many times in a row, which may fail
> > with EAGAIN sometimes. Avoid generating a test failure in this case.
> > 
> > libstdc++-v3/ChangeLog:
> > 
> > 	PR libstdc++/54185
> > 	* testsuite/30_threads/condition_variable/54185.cc: Make test robust
> > 	to random pthread_create() failures.
> 
> Thanks for the patch. It certainly looks reasonable, but I wonder if
> the attached version wouldn't be (very slightly) better. The
> difference is that instead of just giving up at the first EAGAIN we
> keep trying. This way we might be able to create a few more threads
> before the loop finishes. If we still keep failing, it works the same.
>
> I've also added a check that the failures are due to EAGAIN, and we'll
> still terminate if there's some other problem. I'm assuming that your
> failures are EAGAIN. Do you know why that's happening? Does your
> system a low value for RLIMIT_NPROC or something?
>

Right, good point to check for EAGAIN. Yes, that's the error I get. I don't
understand why it happens. It's not related to libstdc++, I can reproduce it
with the below:

======
#include <pthread.h>
void* do_nothing (void*) 
{
  return nullptr;
}
int main () {
  for (int i = 0; i != 1000; ++i)
    {
      for (int j = 0; j != 10; ++j)
	{
	  pthread_t thread;
	  const int err = pthread_create (&thread, nullptr, do_nothing, nullptr);
	  if (err) return 1;
	  pthread_join (thread, nullptr);
        }
    }
}
======

If I run this just once at a time, it never fails. But if I run it twice at
a time, it fails about 30% of the time, like:
root@host:/home/lewis# (./pthread_fail || echo ERR) & \
                       (./pthread_fail || echo ERR) & wait
[1] 25041
[2] 25042
ERR
ERR

All the rlimits are infinite or as high as possible, but I dug around a bit
and it seems this is a systemd thing, this system had systemd-logind
disabled (perhaps not in the correct way) and something about the
configuration led to the issue. Enabling systemd-logind resolves it for
me. So perhaps this was mostly specific to me. Sorry if I wasted your
time... if you still think it's worth doing something here I am happy to
help.

FWIW, regarding your extension to the patch, in case there are some
legitimate thread creation problems, one thing to keep in mind is that the
retrying after failure makes certain things worse. For instance, (with my
system in the previous state), what would happen is the 54185.cc hit the
pthread_create failure, then prior to this patch it just bailed out. With
either of these patches it tries more times, which can worsen issues in
unrelated test cases running in parallel, that may see random failures in
their own forks or thread creations. This test case is trying hard to
reproduce the race condition by running 1000 iterations, which seems
worthwhile given it's still failing on some systems like AIX, but on the
other hand it's possible doing 50 instead of 1000 would work too, and be
less prone to unrelated resource issues.

Thanks for taking a look at this.

-Lewis

> The failures for that testcase on AIX appear to be different. It just
> segfaults after destroying the condition_variable, which probably
> means there's a POSIX conformance issue in AIX's pthread_cond_t.
> 
> 

> diff --git a/libstdc++-v3/testsuite/30_threads/condition_variable/54185.cc b/libstdc++-v3/testsuite/30_threads/condition_variable/54185.cc
> index ea0d5bb8740..8ccb79e6de6 100644
> --- a/libstdc++-v3/testsuite/30_threads/condition_variable/54185.cc
> +++ b/libstdc++-v3/testsuite/30_threads/condition_variable/54185.cc
> @@ -31,31 +31,48 @@
>  std::condition_variable* cond = nullptr;
>  std::mutex mx;
>  int started = 0;
> +bool notified = false;
>  int constexpr NUM_THREADS = 10;
>  
> -void do_thread_a()
> +void do_thread_a(bool wait)
>  {
>    std::unique_lock<std::mutex> lock(mx);
> -  if(++started >= NUM_THREADS)
> +  if (++started >= NUM_THREADS)
>    {
> +    notified = true;
>      cond->notify_all();
>      delete cond;
>      cond = nullptr;
>    }
> -  else
> -    cond->wait(lock);
> +  else if (wait)
> +    cond->wait(lock, [] { return notified; });
>  }
>  
> -int main(){
> +int main()
> +{
>    std::vector<std::thread> vec;
> -  for(int j = 0; j < 1000; ++j)
> +  for (int j = 0; j < 1000; ++j)
>    {
>      started = 0;
> +    notified = false;
>      cond = new std::condition_variable;
>      for (int i = 0; i < NUM_THREADS; ++i)
> -      vec.emplace_back(&do_thread_a);
> -    for (int i = 0; i < NUM_THREADS; ++i)
> -      vec[i].join();
> +      {
> +	try
> +	  {
> +	    vec.emplace_back(&do_thread_a, true);
> +	  }
> +	catch(const std::system_error& e)
> +	  {
> +	    if (e.code() == std::errc::resource_unavailable_try_again)
> +	      // Thread creation may fail due to resource limits; run serially.
> +	      do_thread_a(false);
> +	    else
> +	      throw;
> +	  }
> +      }
> +    for (auto& thread : vec)
> +      thread.join();
>      vec.clear();
>    }
>  }