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From: Valentin Schneider <valentin.schneider@arm.com>
Date: Fri, 13 Nov 2020 11:24:14 +0000
Subject: [PATCH] sched/core: Add missing completion for affine_move_task()
waiters
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/older/patches-5.10.17-rt32.tar.xz
Qian reported that some fuzzer issuing sched_setaffinity() ends up stuck on
a wait_for_completion(). The problematic pattern seems to be:
affine_move_task()
// task_running() case
stop_one_cpu();
wait_for_completion(&pending->done);
Combined with, on the stopper side:
migration_cpu_stop()
// Task moved between unlocks and scheduling the stopper
task_rq(p) != rq &&
// task_running() case
dest_cpu >= 0
=> no complete_all()
This can happen with both PREEMPT and !PREEMPT, although !PREEMPT should
be more likely to see this given the targeted task has a much bigger window
to block and be woken up elsewhere before the stopper runs.
Make migration_cpu_stop() always look at pending affinity requests; signal
their completion if the stopper hits a rq mismatch but the task is
still within its allowed mask. When Migrate-Disable isn't involved, this
matches the previous set_cpus_allowed_ptr() vs migration_cpu_stop()
behaviour.
Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Reported-by: Qian Cai <cai@redhat.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/lkml/8b62fd1ad1b18def27f18e2ee2df3ff5b36d0762.camel@redhat.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/sched/core.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1927,7 +1927,7 @@ static int migration_cpu_stop(void *data
else
p->wake_cpu = dest_cpu;
- } else if (dest_cpu < 0) {
+ } else if (dest_cpu < 0 || pending) {
/*
* This happens when we get migrated between migrate_enable()'s
* preempt_enable() and scheduling the stopper task. At that
@@ -1938,6 +1938,17 @@ static int migration_cpu_stop(void *data
*/
/*
+ * The task moved before the stopper got to run. We're holding
+ * ->pi_lock, so the allowed mask is stable - if it got
+ * somewhere allowed, we're done.
+ */
+ if (pending && cpumask_test_cpu(task_cpu(p), p->cpus_ptr)) {
+ p->migration_pending = NULL;
+ complete = true;
+ goto out;
+ }
+
+ /*
* When this was migrate_enable() but we no longer have an
* @pending, a concurrent SCA 'fixed' things and we should be
* valid again. Nothing to do.
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