[PATCH] sched: Fix race between yield_to() and try_to_wake_up()

[Tianchen Ding]

We met a SCHED_WARN in set_next_buddy():
  __warn_printk
  set_next_buddy
  yield_to_task_fair
  yield_to
  kvm_vcpu_yield_to [kvm]
  ...

After a short dig, we found the rq_lock held by yield_to() may not
be exactly the rq that the target task belongs to. There is a race
window against try_to_wake_up().

         CPU0                             target_task

                                        blocking on CPU1
   lock rq0 & rq1
   double check task_rq == p_rq, ok
                                        woken to CPU2 (lock task_pi & rq2)
                                        task_rq = rq2
   yield_to_task_fair (w/o lock rq2)

In this race window, yield_to() is operating the task w/o the currect
lock. Fix this by taking task pi_lock first.

Fixes: d95f41220065 ("sched: Add yield_to(task, preempt) functionality")
Signed-off-by: Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx>
---
 kernel/sched/syscalls.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index ff0e5ab4e37c..943406c4ee86 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -1433,7 +1433,7 @@ int __sched yield_to(struct task_struct *p, bool preempt)
 	struct rq *rq, *p_rq;
 	int yielded = 0;
 
-	scoped_guard (irqsave) {
+	scoped_guard (raw_spinlock_irqsave, &p->pi_lock) {
 		rq = this_rq();
 
 again:
-- 
2.39.3