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Subject[PATCH 4.18 039/123] stop_machine: Reflow cpu_stop_queue_two_works()
Date
4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@infradead.org>

commit b80a2bfce85e1051056d98d04ecb2d0b55cbbc1c upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/stop_machine.c | 41 +++++++++++++++++++++++------------------
1 file changed, 23 insertions(+), 18 deletions(-)

--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -236,13 +236,24 @@ static int cpu_stop_queue_two_works(int
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;
}

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