[patch-rt] locking, rwlock-rt: do not save state multiple times in __write_rt_lock()

[Mike Galbraith]

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 <efa...@gmx.de>
---
 kernel/locking/rwlock-rt.c |   37 ++++++++++++++++++++-----------------
 1 file changed, 20 insertions(+), 17 deletions(-)

--- a/kernel/locking/rwlock-rt.c
+++ b/kernel/locking/rwlock-rt.c
@@ -190,30 +190,33 @@ 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(&self->pi_lock);
-               self->saved_state = self->state;
-               __set_current_state_no_track(TASK_UNINTERRUPTIBLE);
-               raw_spin_unlock(&self->pi_lock);
+       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);
 
+       for (;;) {
                /* Have all readers left the critical region? */
-               if (!atomic_read(&lock->readers)) {
-                       atomic_set(&lock->readers, WRITER_BIAS);
-                       raw_spin_lock(&self->pi_lock);
-                       __set_current_state_no_track(self->saved_state);
-                       self->saved_state = TASK_RUNNING;
-                       raw_spin_unlock(&self->pi_lock);
-                       raw_spin_unlock_irqrestore(&m->wait_lock, flags);
-                       return;
-               }
+               if (!atomic_read(&lock->readers))
+                       break;
 
                raw_spin_unlock_irqrestore(&m->wait_lock, flags);
-
                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);
        }
+
+       atomic_set(&lock->readers, WRITER_BIAS);
+       raw_spin_lock(&self->pi_lock);
+       __set_current_state_no_track(self->saved_state);
+       self->saved_state = TASK_RUNNING;
+       raw_spin_unlock(&self->pi_lock);
+       raw_spin_unlock_irqrestore(&m->wait_lock, flags);
 }
 
 int __write_rt_trylock(struct rt_rw_lock *lock)