From: Mike Galbraith <efault@gmx.de>
Date: Fri, 18 Aug 2017 10:56:14 +0200
Subject: locking, rwlock-rt: do not save state multiple times in __write_rt_lock()
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: 6a3e7e316c9cb3ef223528de916b625fe6c5532e
Patch-mainline: Queued in subsystem maintainer repository
References: SLE15 Realtime Extension

Save state prior to entering the acquisition loop, otherwise we may
initially see readers, but upon releasing ->wait_lock see none, loop
back around, and having not slept, save TASK_UNINTERRUPTIBLE.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 kernel/locking/rwlock-rt.c |   18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

--- a/kernel/locking/rwlock-rt.c
+++ b/kernel/locking/rwlock-rt.c
@@ -190,14 +190,14 @@ void __sched __write_rt_lock(struct rt_r
 	/* Force readers into slow path */
 	atomic_sub(READER_BIAS, &lock->readers);
 
-	for (;;) {
-		raw_spin_lock_irqsave(&m->wait_lock, flags);
+	raw_spin_lock_irqsave(&m->wait_lock, flags);
 
-		raw_spin_lock(&self->pi_lock);
-		self->saved_state = self->state;
-		__set_current_state_no_track(TASK_UNINTERRUPTIBLE);
-		raw_spin_unlock(&self->pi_lock);
+	raw_spin_lock(&self->pi_lock);
+	self->saved_state = self->state;
+	__set_current_state_no_track(TASK_UNINTERRUPTIBLE);
+	raw_spin_unlock(&self->pi_lock);
 
+	for (;;) {
 		/* Have all readers left the critical region? */
 		if (!atomic_read(&lock->readers)) {
 			atomic_set(&lock->readers, WRITER_BIAS);
@@ -213,6 +213,12 @@ void __sched __write_rt_lock(struct rt_r
 
 		if (atomic_read(&lock->readers) != 0)
 			schedule();
+
+		raw_spin_lock_irqsave(&m->wait_lock, flags);
+
+		raw_spin_lock(&self->pi_lock);
+		__set_current_state_no_track(TASK_UNINTERRUPTIBLE);
+		raw_spin_unlock(&self->pi_lock);
 	}
 }