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], isaacm@xxxxxxxxxxxxxx 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 <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
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;
}
