From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 8 Mar 2017 14:23:35 +0100
Subject: futex: workaround migrate_disable/enable in different context
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: aed0f50e58eb22e9636d6e200e5007a7e240a448
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

migrate_disable()/migrate_enable() takes a different path in atomic() vs
!atomic() context. These little hacks ensure that we don't underflow / overflow
the migrate code counts properly while we lock the hb lockwith interrupts
enabled and unlock it with interrupts disabled.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 kernel/futex.c |   19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2803,6 +2803,14 @@ retry_private:
 	 * before __rt_mutex_start_proxy_lock() is done.
 	 */
 	raw_spin_lock_irq(&q.pi_state->pi_mutex.wait_lock);
+	/*
+	 * the migrate_disable() here disables migration in the in_atomic() fast
+	 * path which is enabled again in the following spin_unlock(). We have
+	 * one migrate_disable() pending in the slow-path which is reversed
+	 * after the raw_spin_unlock_irq() where we leave the atomic context.
+	 */
+	migrate_disable();
+
 	spin_unlock(q.lock_ptr);
 	/*
 	 * __rt_mutex_start_proxy_lock() unconditionally enqueues the @rt_waiter
@@ -2811,6 +2819,7 @@ retry_private:
 	 */
 	ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
 	raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);
+	migrate_enable();
 
 	if (ret) {
 		if (ret == 1)
@@ -2956,11 +2965,21 @@ retry:
 		 * rt_waiter. Also see the WARN in wake_futex_pi().
 		 */
 		raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
+		/*
+		 * Magic trickery for now to make the RT migrate disable
+		 * logic happy. The following spin_unlock() happens with
+		 * interrupts disabled so the internal migrate_enable()
+		 * won't undo the migrate_disable() which was issued when
+		 * locking hb->lock.
+		 */
+		migrate_disable();
 		spin_unlock(&hb->lock);
 
 		/* drops pi_state->pi_mutex.wait_lock */
 		ret = wake_futex_pi(uaddr, uval, pi_state);
 
+		migrate_enable();
+
 		put_pi_state(pi_state);
 
 		/*