Messages in this thread Patch in this message | | | Date | Sun, 30 Jul 2006 08:36:05 +0400 | From | Oleg Nesterov <> | Subject | rt_mutex_timed_lock() vs hrtimer_wakeup() race ? |
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I am trying to get some understanding of rt_mutex, but I'm afraid it's not possible without your help...
// runs on CPU 0 rt_mutex_slowlock:
// main loop for (;;) { ...
if (timeout && !timeout->task) { ret = -ETIMEDOUT; break; }
...
schedule();
...
set_current_state(state); }
What if timeout->timer is fired on CPU 1 right before set_current_state() ?
hrtimer_wakeup() does:
timeout->task = NULL; <----- [1]
spin_lock(runqueues->lock);
task->state = TASK_RUNNING; <----- [2]
(task->array != NULL, so try_to_wake_up() just goes to out_running)
If my understanding correct, [1] may slip into the critical section (because spin_lock() is not a wmb), so that CPU 0 will see [2] but not [1]. In that case rt_mutex_slowlock() can miss the timeout (set_current_state()->mb() doesn't help).
Of course, this race (even _if_ I am right) is pure theoretical, but probably we need smp_wmb() after [1] in hrtimer_wakeup().
Note that do_nanosleep() is ok, hrtimer_base->lock provides a necessary serialization. In fact, I think it can use __set_current_state(), because both hrtimer_start() and run_hrtimer_queue() do lock/unlock of base->lock.
Another question, task_blocks_on_rt_mutex() does get_task_struct() and checks owner->pi_blocked_on != NULL under owner->pi_lock. Why ? RT_MUTEX_HAS_WAITERS bit is set, we are holding ->wait_lock, so the 'owner' can't go away until we drop ->wait_lock. I think we can drop owner->pi_lock right after __rt_mutex_adjust_prio(owner), we can't miss owner->pi_blocked_on != NULL if it was true before we take owner->pi_lock, and this is the case we should worry about, yes?
In other words (because I myself can't parse the paragraph above :), could you explain me why this patch is not correct:
--- rtmutex.c~ 2006-07-30 05:15:38.000000000 +0400 +++ rtmutex.c 2006-07-30 05:41:44.000000000 +0400 @@ -407,7 +407,7 @@ static int task_blocks_on_rt_mutex(struc struct task_struct *owner = rt_mutex_owner(lock); struct rt_mutex_waiter *top_waiter = waiter; unsigned long flags; - int boost = 0, res; + int res; spin_lock_irqsave(¤t->pi_lock, flags); __rt_mutex_adjust_prio(current); @@ -431,24 +431,20 @@ static int task_blocks_on_rt_mutex(struc plist_add(&waiter->pi_list_entry, &owner->pi_waiters); __rt_mutex_adjust_prio(owner); - if (owner->pi_blocked_on) { - boost = 1; - /* gets dropped in rt_mutex_adjust_prio_chain()! */ - get_task_struct(owner); - } spin_unlock_irqrestore(&owner->pi_lock, flags); + + if (owner->pi_blocked_on) + goto boost; } else if (debug_rt_mutex_detect_deadlock(waiter, detect_deadlock)) { - spin_lock_irqsave(&owner->pi_lock, flags); - if (owner->pi_blocked_on) { - boost = 1; - /* gets dropped in rt_mutex_adjust_prio_chain()! */ - get_task_struct(owner); - } - spin_unlock_irqrestore(&owner->pi_lock, flags); + if (owner->pi_blocked_on) + goto boost; } - if (!boost) - return 0; + + return 0; +boost: + /* gets dropped in rt_mutex_adjust_prio_chain()! */ + get_task_struct(owner); spin_unlock(&lock->wait_lock); ---------------------------------------------------------- The same question for remove_waiter()/rt_mutex_adjust_pi().
The last (stupid) one, wake_up_new_task:
if (unlikely(!current->array)) __activate_task(p, rq); (offtopic) Is it really possible to have current->array == NULL here?
else { p->prio = current->prio; What if current was pi-boosted so that rt_prio(current->prio) == 1, who will de-boost the child?
p->normal_prio = current->normal_prio;
Why? p->normal_prio was calculated by effective_prio() above, could you explain why that value is not ok?
Thanks,
Oleg.
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