With proxy-exec once we do return migration from ttwu(), if a
task is proxying for a waiting donor, and the donor is woken up,
we switch the rq->donor to point to idle briefly until we can
re-enter __schedule().

However, if a task that was acting as a proxy calls into
yield() right after the donor is switched to idle, it may
trip a null  pointer traversal, because the idle task doesn't
have a yield_task() pointer.

So add a conditional to ensure we don't try to call the
yield_task() pointer in that case.

This was only recently found because prior to commit
127b90315ca07 ("sched/proxy: Yield the donor task")
do_sched_yield() incorrectly called
current->sched_class_yield_task() instead of using
rq->donor.

Signed-off-by: John Stultz <jstultz@google.com>
---
Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com
---
 kernel/sched/syscalls.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index bf360a6fbb800..4b2b81437b03b 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -1351,7 +1351,8 @@ static void do_sched_yield(void)
 	rq = this_rq_lock_irq(&rf);
 
 	schedstat_inc(rq->yld_count);
-	rq->donor->sched_class->yield_task(rq);
+	if (rq->donor->sched_class->yield_task)
+		rq->donor->sched_class->yield_task(rq);
 
 	preempt_disable();
 	rq_unlock_irq(rq, &rf);
-- 
2.52.0.487.g5c8c507ade-goog