On Mon, Jul 30, 2018 at 12:20:57PM +0200, Thomas Gleixner wrote:
> On Tue, 24 Jul 2018, Sebastian Andrzej Siewior wrote:
> > On 2018-07-23 18:13:48 [-0700], isa...@codeaurora.org wrote:
> > > Hi all,
> > Hi,
> > 
> > > Are there any comments about this patch?
> > 
> > I haven't look in detail at this but your new preempt_disable() makes
> > things unbalanced for the err != 0 case.
> 
> It doesn't but that code is really an unreadable pile of ...

---
Subject: 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 <t...@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
 kernel/stop_machine.c | 41 +++++++++++++++++++++++------------------
 1 file 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 @@ static int cpu_stop_queue_two_works(int cpu1, struct 
cpu_stop_work *work1,
         * 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;
 }

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