FIFO/Pipe Implementation

Sebastian Huber sebastian.huber at
Thu Apr 12 12:30:36 UTC 2012


does anyone know why the FIFO/pipe implementation uses barriers for 
synchronization?  For me the usage of the barrier looks like a hand crafted 
condition variable.

I think there is also a race condition in its use.  The rtems_barrier_release() 
releases only the threads currently waiting at the barrier.  Now we have 
several code sections like this:

       if (! PIPE_WRITEWAIT(pipe))
         ret = -EINTR;

Thus the release may have no effect in case it happens after the PIPE_UNLOCK 
and before the PIPE_WRITEWAIT (which is a rtems_barrier_wait() with no time out).

The bdbuf code uses a similar approach with semaphores.  It disables the 
preemption to avoid this race condition:

static void
rtems_bdbuf_anonymous_wait (rtems_bdbuf_waiters *waiters)
   rtems_status_code sc;
   rtems_mode        prev_mode;

    * Indicate we are waiting.

    * Disable preemption then unlock the cache and block.  There is no POSIX
    * condition variable in the core API so this is a work around.
    * The issue is a task could preempt after the cache is unlocked because it is
    * blocking or just hits that window, and before this task has blocked on the
    * semaphore. If the preempting task flushes the queue this task will not see
    * the flush and may block for ever or until another transaction flushes this
    * semaphore.
   prev_mode = rtems_bdbuf_disable_preemption ();

    * Unlock the cache, wait, and lock the cache when we return.
   rtems_bdbuf_unlock_cache ();

   sc = rtems_semaphore_obtain (waiters->sema, RTEMS_WAIT, 

   if (sc == RTEMS_TIMEOUT)
     rtems_fatal_error_occurred (RTEMS_BLKDEV_FATAL_BDBUF_CACHE_WAIT_TO);

     rtems_fatal_error_occurred (RTEMS_BLKDEV_FATAL_BDBUF_CACHE_WAIT_2);

   rtems_bdbuf_lock_cache ();

   rtems_bdbuf_restore_preemption (prev_mode);


Please note that this approach is not SMP proof.  I think we need condition 
variables in the RTEMS Classic API.

Sebastian Huber, embedded brains GmbH

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