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Subject[PATCH v2] sched/fair: WARN and refuse to set buddy when !se->on_rq
Date
If we set a next or last buddy for a se that is not on_rq, we will
end up taking a NULL pointer dereference in wakeup_preempt_entity
via pick_next_task_fair.

Detect when we would be about to do that, throw a warning and
then refuse to actually set it.

This has been suggested at least twice[0][1]: just do it.

[0] https://marc.info/?l=linux-kernel&m=146651668921468&w=2
[1] https://lkml.org/lkml/2016/6/16/663

Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Ben Segall <bsegall@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Daniel Axtens <dja@axtens.net>

---

I recently had to debug a problem with these (we hadn't backported
Konstantin's patches in this area) and this would have saved a lot
of time/pain.

v2: use SCHED_WARN_ON to restrict when the test is run. This is a
macro for WARN_ON_ONCE, which is convenient.
---
kernel/sched/fair.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d71109321841..44b94cfe02cb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6168,8 +6168,11 @@ static void set_last_buddy(struct sched_entity *se)
if (entity_is_task(se) && unlikely(task_of(se)->policy == SCHED_IDLE))
return;

- for_each_sched_entity(se)
+ for_each_sched_entity(se) {
+ if (SCHED_WARN_ON(!se->on_rq))
+ return;
cfs_rq_of(se)->last = se;
+ }
}

static void set_next_buddy(struct sched_entity *se)
@@ -6177,8 +6180,11 @@ static void set_next_buddy(struct sched_entity *se)
if (entity_is_task(se) && unlikely(task_of(se)->policy == SCHED_IDLE))
return;

- for_each_sched_entity(se)
+ for_each_sched_entity(se) {
+ if (SCHED_WARN_ON(!se->on_rq))
+ return;
cfs_rq_of(se)->next = se;
+ }
}

static void set_skip_buddy(struct sched_entity *se)
--
2.11.0
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