Add a separate local variable for the boost/deboost logic to make the code more readable. Add comments where appropriate. Signed-off-by: Thomas Gleixner Cc: Peter Zijlstra Cc: Steven Rostedt Cc: Lai Jiangshan Link: http://lkml.kernel.org/r/20140522031950.168005692@linutronix.de Signed-off-by: Thomas Gleixner --- kernel/locking/rtmutex.c | 51 +++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 43 insertions(+), 8 deletions(-) Index: tip/kernel/locking/rtmutex.c =================================================================== --- tip.orig/kernel/locking/rtmutex.c +++ tip/kernel/locking/rtmutex.c @@ -303,10 +303,11 @@ static int rt_mutex_adjust_prio_chain(st struct rt_mutex_waiter *orig_waiter, struct task_struct *top_task) { - struct rt_mutex *lock; struct rt_mutex_waiter *waiter, *top_waiter = orig_waiter; + struct rt_mutex_waiter *prerequeue_top_waiter; enum rtmutex_chainwalk detect_deadlock; int ret = 0, depth = 0; + struct rt_mutex *lock; unsigned long flags; detect_deadlock = rt_mutex_cond_detect_deadlock(orig_waiter, @@ -381,6 +382,11 @@ static int rt_mutex_adjust_prio_chain(st if (!detect_deadlock && waiter->prio == task->prio) goto out_unlock_pi; + /* + * We need to trylock here as we are holding task->pi_lock, + * which is the reverse lock order versus the other rtmutex + * operations. + */ lock = waiter->lock; if (!raw_spin_trylock(&lock->wait_lock)) { raw_spin_unlock_irqrestore(&task->pi_lock, flags); @@ -401,7 +407,11 @@ static int rt_mutex_adjust_prio_chain(st goto out_unlock_pi; } - top_waiter = rt_mutex_top_waiter(lock); + /* + * Store the top waiter before doing any requeue operation. We + * need it for the boost/deboost decision below. + */ + prerequeue_top_waiter = rt_mutex_top_waiter(lock); /* Requeue the waiter */ rt_mutex_dequeue(lock, waiter); @@ -410,13 +420,18 @@ static int rt_mutex_adjust_prio_chain(st /* Release the task */ raw_spin_unlock_irqrestore(&task->pi_lock, flags); + + /* + * We must abort the chain walk if there is no lock owner even + * in the dead lock detection case, as we have nothing to + * follow here. This is the end of the chain we are walking. + */ if (!rt_mutex_owner(lock)) { /* * If the requeue above changed the top waiter, then we need * to wake the new top waiter up to try to get the lock. */ - - if (top_waiter != rt_mutex_top_waiter(lock)) + if (prerequeue_top_waiter != rt_mutex_top_waiter(lock)) wake_up_process(rt_mutex_top_waiter(lock)->task); raw_spin_unlock(&lock->wait_lock); goto out_put_task; @@ -429,17 +444,32 @@ static int rt_mutex_adjust_prio_chain(st raw_spin_lock_irqsave(&task->pi_lock, flags); if (waiter == rt_mutex_top_waiter(lock)) { - /* Boost the owner */ - rt_mutex_dequeue_pi(task, top_waiter); + /* + * The waiter became the new top (highest priority) + * waiter on the lock. Replace the previous top waiter + * in the owner tasks pi waiters list with this waiter. + */ + rt_mutex_dequeue_pi(task, prerequeue_top_waiter); rt_mutex_enqueue_pi(task, waiter); __rt_mutex_adjust_prio(task); - } else if (top_waiter == waiter) { - /* Deboost the owner */ + } else if (prerequeue_top_waiter == waiter) { + /* + * The waiter was the top waiter on the lock, but is + * no longer the top prority waiter. Replace waiter in + * the owner tasks pi waiters list with the new top + * (highest priority) waiter. + */ rt_mutex_dequeue_pi(task, waiter); waiter = rt_mutex_top_waiter(lock); rt_mutex_enqueue_pi(task, waiter); __rt_mutex_adjust_prio(task); + + } else { + /* + * Nothing changed. No need to do any priority + * adjustment. + */ } raw_spin_unlock_irqrestore(&task->pi_lock, flags); @@ -447,6 +477,11 @@ static int rt_mutex_adjust_prio_chain(st top_waiter = rt_mutex_top_waiter(lock); raw_spin_unlock(&lock->wait_lock); + /* + * If the current waiter is not the top waiter on the lock, + * then we can stop the chain walk here if we are not in full + * deadlock detection mode. + */ if (!detect_deadlock && waiter != top_waiter) goto out_put_task; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/