linux/debian/patches-rt/0132-futex-Ensure-lock-unlock-symetry-versus-pi_lock-and-.patch

From: Thomas Gleixner <tglx@linutronix.de>
Date: Fri, 1 Mar 2013 11:17:42 +0100
Subject: [PATCH 132/290] futex: Ensure lock/unlock symetry versus pi_lock and
 hash bucket lock
Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=653ee648a2ba5f857ec351405fc64f9cf8867b84

In exit_pi_state_list() we have the following locking construct:

   spin_lock(&hb->lock);
   raw_spin_lock_irq(&curr->pi_lock);

   ...
   spin_unlock(&hb->lock);

In !RT this works, but on RT the migrate_enable() function which is
called from spin_unlock() sees atomic context due to the held pi_lock
and just decrements the migrate_disable_atomic counter of the
task. Now the next call to migrate_disable() sees the counter being
negative and issues a warning. That check should be in
migrate_enable() already.

Fix this by dropping pi_lock before unlocking hb->lock and reaquire
pi_lock after that again. This is safe as the loop code reevaluates
head again under the pi_lock.

Reported-by: Yong Zhang <yong.zhang@windriver.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/futex.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index b2a90c66d8f4..4d6501d689b5 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -918,7 +918,9 @@ void exit_pi_state_list(struct task_struct *curr)
 		if (head->next != next) {
 			/* retain curr->pi_lock for the loop invariant */
 			raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
+			raw_spin_unlock_irq(&curr->pi_lock);
 			spin_unlock(&hb->lock);
+			raw_spin_lock_irq(&curr->pi_lock);
 			put_pi_state(pi_state);
 			continue;
 		}