Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932349AbaFIU2S (ORCPT ); Mon, 9 Jun 2014 16:28:18 -0400 Received: from www.linutronix.de ([62.245.132.108]:46577 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754097AbaFIU2L (ORCPT ); Mon, 9 Jun 2014 16:28:11 -0400 Message-Id: <20140609202336.068962483@linutronix.de> User-Agent: quilt/0.63-1 Date: Mon, 09 Jun 2014 20:28:07 -0000 From: Thomas Gleixner To: LKML Cc: Steven Rostedt , Peter Zijlstra , Ingo Molnar , Lai Jiangshan , Jason Low , Brad Mouring Subject: [patch V3 2/7] rtmutex: Clarify the boost/deboost part References: <20140609201118.387571774@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Disposition: inline; filename=rtmutex-clarify-the-lock-chain-walk.patch X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- kernel/locking/rtmutex.c | 58 ++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 48 insertions(+), 10 deletions(-) Index: tip/kernel/locking/rtmutex.c =================================================================== --- tip.orig/kernel/locking/rtmutex.c +++ tip/kernel/locking/rtmutex.c @@ -293,9 +293,10 @@ 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; int detect_deadlock, ret = 0, depth = 0; + struct rt_mutex *lock; unsigned long flags; detect_deadlock = debug_rt_mutex_detect_deadlock(orig_waiter, @@ -402,9 +403,14 @@ static int rt_mutex_adjust_prio_chain(st goto out_unlock_pi; } - top_waiter = rt_mutex_top_waiter(lock); + /* + * Store the current top waiter before doing the requeue + * operation on @lock. We need it for the boost/deboost + * decision below. + */ + prerequeue_top_waiter = rt_mutex_top_waiter(lock); - /* Requeue the waiter */ + /* Requeue the waiter in the lock waiter list. */ rt_mutex_dequeue(lock, waiter); waiter->prio = task->prio; rt_mutex_enqueue(lock, waiter); @@ -413,35 +419,58 @@ static int rt_mutex_adjust_prio_chain(st raw_spin_unlock_irqrestore(&task->pi_lock, flags); put_task_struct(task); + /* + * 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); return 0; } - /* Grab the next task */ + /* Grab the next task, i.e. the owner of @lock */ task = rt_mutex_owner(lock); get_task_struct(task); 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 + * and adjust the priority of the owner. + */ + 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 and adjust the priority + * of the owner. + * The new top waiter is stored in @waiter so that + * @waiter == @top_waiter evaluates to true below and + * we continue to deboost the rest of the chain. + */ 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. + */ } /* @@ -454,6 +483,10 @@ static int rt_mutex_adjust_prio_chain(st raw_spin_unlock_irqrestore(&task->pi_lock, flags); + /* + * Store the top waiter of @lock for the end of chain walk + * decision below. + */ top_waiter = rt_mutex_top_waiter(lock); raw_spin_unlock(&lock->wait_lock); @@ -464,6 +497,11 @@ static int rt_mutex_adjust_prio_chain(st if (!next_lock) goto out_put_task; + /* + * 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/