From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 8 Mar 2017 14:23:35 +0100
Subject: [PATCH] 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: da77ceac3d20f27310a07a7c346a4ee6b40d6c28
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

migrate_enable() invokes __schedule() and it expects a preempt count of one.
Holding a raw_spinlock_t with disabled interrupts should not allow scheduling.

These little hacks ensure that we don't schedule while we lock the hb lockwith
interrupts enabled and unlock it with interrupts disabled.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[XXX: As per PeterZ suggesstion
	set_thread_flag(TIF_NEED_RESCHED); preempt_fold_need_resched()
 would trigger a scheduler invocation on the last preempt_enable() which in
 turn would allow to drop this.
]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
---
 kernel/futex.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2980,6 +2980,14 @@ static int futex_lock_pi(u32 __user *uad
 	 * 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
@@ -2988,6 +2996,7 @@ static int futex_lock_pi(u32 __user *uad
 	 */
 	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)
@@ -3136,10 +3145,19 @@ static int futex_unlock_pi(u32 __user *u
 		 * 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);