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From: "jsm28 at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sources.redhat.com
Subject: [Bug nptl/14532] New: Generic sem_post race
Date: Thu, 30 Aug 2012 19:14:00 -0000
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http://sourceware.org/bugzilla/show_bug.cgi?id=14532

             Bug #: 14532
           Summary: Generic sem_post race
           Product: glibc
           Version: 2.16
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nptl
        AssignedTo: unassigned@sourceware.org
        ReportedBy: jsm28@gcc.gnu.org
                CC: drepper.fsp@gmail.com
    Classification: Unclassified


The generic Linux version of sem_post (nptl/sysdeps/unix/sysv/linux/sem_post.c)
uses atomic_compare_and_exchange_bool_acq to increment the semaphore.  This is
an unlock operation and should actually be using a release barrier.  The
following test fails on some MIPS systems (MIPS uses the generic version of
sem_post where various architectures use their own .S versions).  Testing a
patch.

#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>

#define NTHREADS 10
#define NITER 100000

sem_t sem;
int c;
volatile int thread_fail;

static void *
tf (void *arg)
{
  for (int i = 0; i < NITER; i++)
    {
      if (sem_wait (&sem) != 0)
        {
          perror ("sem_wait");
          thread_fail = 1;
        }
      ++c;
      if (sem_post (&sem) != 0)
        {
          perror ("sem_post");
          thread_fail = 1;
        }
    }
  return NULL;
}

int
main (void)
{
  if (sem_init (&sem, 0, 0) != 0)
    {
      perror ("sem_init");
      return 1;
    }

  pthread_t th[NTHREADS];
  for (int i = 0; i < NTHREADS; i++)
    {
      if (pthread_create (&th[i], NULL, tf, NULL) != 0)
        {
          puts ("pthread_create failed");
          return 1;
        }
    }

  if (sem_post (&sem) != 0)
    {
      perror ("sem_post");
      return 1;
    }

  for (int i = 0; i < NTHREADS; i++)
    if (pthread_join (th[i], NULL) != 0)
      {
        puts ("pthread_join failed");
        return 1;
      }

  if (c != NTHREADS * NITER)
    {
      printf ("c = %d, should be %d\n", c, NTHREADS * NITER);
      return 1;
    }
  return thread_fail;
}

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