Wind River Support Network

Description

I found a problem related to rtmutex which may introduce a deadlock.

The problem is in wakeup_next_waiter() as follows:

static void wakeup_next_waiter(struct rt_mutex *lock)
{
        ...
        rt_mutex_wake_waiter(waiter);
        wake_up_process(waiter->task);
}

I think last calling of wake_up_process() is not necessary because waiter->task is already woken up in rt_mutex_wake_waiter().  To make matters worse, I think the calling of wake_up_process() may introduce a deadlock.  The scenario of the deadlock is as follows:

CPU0                                                    CPU1
====================================================================================================
                                                        rt_spin_lock_slowlock
                                                          schedule_rt_mutex(lock);
                                                          <sleeping...>

rt_spin_lock_slowunlock
  raw_spin_lock(&lock->wait_lock)
  __rt_spin_lock_slowunlock(lock)
    wakeup_next_waiter(lock)
      ...
      rt_mutex_wake_waiter(waiter);
        wake_up_lock_sleeper(waiter->task);
          try_to_wake_up(p, TASK_ALL,
                         WF_LOCK_SLEEPER);
            ...
            while (p->on_cpu)
              cpu_relax();
            ...
            ttwu_queue(p, cpu); -----------------------> <waked up>
                                                         raw_spin_lock(&lock->wait_lock);
                                                           <spinning...>
      wake_up_process(waiter->task);
        try_to_wake_up(p, TASK_NORMAL, 0);
          while (p->on_cpu)
            cpu_relax();
            <spinning>

    raw_spin_unlock(&lock->wait_lock);

In upstream linux-rt-devel tree, there is no calling of last wake_up_process() in wakeup_next_waiter().

Please check it, and let me know if my understanding is wrong.

Other Downloads

• Wind River Linux 6.0.0.31
• Wind River Linux 6.0.0.32
• Wind River Linux 6.0.0.33
• Wind River Linux 6.0.0.34
• Wind River Linux 6.0.0.35
• Wind River Linux 6.0.0.36
• Wind River Linux 6.0.0.37
• Wind River Linux 6.0.0.38