linux-kernel - [PATCH RT 10/16] Revert "futex: Fix bug on when a requeued RT task times out"

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Date:   Fri, 19 Jul 2019 17:49:41 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        John Kacur <jkacur@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Julia Cartwright <julia@...com>,
        Daniel Wagner <wagi@...om.org>, tom.zanussi@...ux.intel.com
Subject: [PATCH RT 10/16] Revert "futex: Fix bug on when a requeued RT task times out"

4.19.59-rt24-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

[ Upstream commit f1a170cb3289a48df26cae3c60d77608f7a988bb ]

Drop the RT fixup, the futex code will be changed to avoid the need for
the workaround.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 kernel/locking/rtmutex.c        | 31 +------------------------------
 kernel/locking/rtmutex_common.h |  1 -
 2 files changed, 1 insertion(+), 31 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 2a9bf2443acc..7f6f402e04ae 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -144,8 +144,7 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
 
 static int rt_mutex_real_waiter(struct rt_mutex_waiter *waiter)
 {
-	return waiter && waiter != PI_WAKEUP_INPROGRESS &&
-		waiter != PI_REQUEUE_INPROGRESS;
+	return waiter && waiter != PI_WAKEUP_INPROGRESS;
 }
 
 /*
@@ -2350,34 +2349,6 @@ int __rt_mutex_start_proxy_lock(struct rt_mutex *lock,
 	if (try_to_take_rt_mutex(lock, task, NULL))
 		return 1;
 
-#ifdef CONFIG_PREEMPT_RT_FULL
-	/*
-	 * In PREEMPT_RT there's an added race.
-	 * If the task, that we are about to requeue, times out,
-	 * it can set the PI_WAKEUP_INPROGRESS. This tells the requeue
-	 * to skip this task. But right after the task sets
-	 * its pi_blocked_on to PI_WAKEUP_INPROGRESS it can then
-	 * block on the spin_lock(&hb->lock), which in RT is an rtmutex.
-	 * This will replace the PI_WAKEUP_INPROGRESS with the actual
-	 * lock that it blocks on. We *must not* place this task
-	 * on this proxy lock in that case.
-	 *
-	 * To prevent this race, we first take the task's pi_lock
-	 * and check if it has updated its pi_blocked_on. If it has,
-	 * we assume that it woke up and we return -EAGAIN.
-	 * Otherwise, we set the task's pi_blocked_on to
-	 * PI_REQUEUE_INPROGRESS, so that if the task is waking up
-	 * it will know that we are in the process of requeuing it.
-	 */
-	raw_spin_lock(&task->pi_lock);
-	if (task->pi_blocked_on) {
-		raw_spin_unlock(&task->pi_lock);
-		return -EAGAIN;
-	}
-	task->pi_blocked_on = PI_REQUEUE_INPROGRESS;
-	raw_spin_unlock(&task->pi_lock);
-#endif
-
 	/* We enforce deadlock detection for futexes */
 	ret = task_blocks_on_rt_mutex(lock, waiter, task,
 				      RT_MUTEX_FULL_CHAINWALK);
diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index 546aaf058b9e..a501f3b47081 100644
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -133,7 +133,6 @@ enum rtmutex_chainwalk {
  * PI-futex support (proxy locking functions, etc.):
  */
 #define PI_WAKEUP_INPROGRESS	((struct rt_mutex_waiter *) 1)
-#define PI_REQUEUE_INPROGRESS	((struct rt_mutex_waiter *) 2)
 
 extern struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock);
 extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
-- 
2.20.1