stop_machine: Reflow cpu_stop_queue_two_works()

The code flow in cpu_stop_queue_two_works() is a little arcane; fix this by
lifting the preempt_disable() to the top to create more natural nesting wrt
the spinlocks and make the wake_up_q() and preempt_enable() unconditional
at the end.

Furthermore, enable preemption in the -EDEADLK case, such that we spin-wait
with preemption enabled.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: isaacm@codeaurora.org
Cc: matt@codeblueprint.co.uk
Cc: psodagud@codeaurora.org
Cc: gregkh@linuxfoundation.org
Cc: pkondeti@codeaurora.org
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180730112140.GH2494@hirez.programming.kicks-ass.net

1 files changed, 23 insertions, 18 deletions

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index e190d1ef3a23..34b6652e8677 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -236,13 +236,24 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
         struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
         DEFINE_WAKE_Q(wakeq);
         int err;
+
 retry:
+        /*
+         * The waking up of stopper threads has to happen in the same
+         * scheduling context as the queueing.  Otherwise, there is a
+         * possibility of one of the above stoppers being woken up by another
+         * CPU, and preempting us. This will cause us to not wake up the other
+         * stopper forever.
+         */
+        preempt_disable();
         raw_spin_lock_irq(&stopper1->lock);
         raw_spin_lock_nested(&stopper2->lock, SINGLE_DEPTH_NESTING);
 
-        err = -ENOENT;
-        if (!stopper1->enabled || !stopper2->enabled)
+        if (!stopper1->enabled || !stopper2->enabled) {
+                err = -ENOENT;
                 goto unlock;
+        }
+
         /*
          * Ensure that if we race with __stop_cpus() the stoppers won't get
          * queued up in reverse order leading to system deadlock.
@@ -253,36 +264,30 @@ retry:
          * It can be falsely true but it is safe to spin until it is cleared,
          * queue_stop_cpus_work() does everything under preempt_disable().
          */
-        err = -EDEADLK;
-        if (unlikely(stop_cpus_in_progress))
-                        goto unlock;
+        if (unlikely(stop_cpus_in_progress)) {
+                err = -EDEADLK;
+                goto unlock;
+        }
 
         err = 0;
         __cpu_stop_queue_work(stopper1, work1, &wakeq);
         __cpu_stop_queue_work(stopper2, work2, &wakeq);
-        /*
-         * The waking up of stopper threads has to happen
-         * in the same scheduling context as the queueing.
-         * Otherwise, there is a possibility of one of the
-         * above stoppers being woken up by another CPU,
-         * and preempting us. This will cause us to n ot
-         * wake up the other stopper forever.
-         */
-        preempt_disable();
+
 unlock:
         raw_spin_unlock(&stopper2->lock);
         raw_spin_unlock_irq(&stopper1->lock);
 
         if (unlikely(err == -EDEADLK)) {
+                preempt_enable();
+
                 while (stop_cpus_in_progress)
                         cpu_relax();
+
                 goto retry;
         }
 
-        if (!err) {
-                wake_up_q(&wakeq);
-                preempt_enable();
-        }
+        wake_up_q(&wakeq);
+        preempt_enable();
 
         return err;
 }