[tip:sched/core] sched/fair: WARN() and refuse to set buddy when !se->on_rq

[tip-bot for Daniel Axtens]

Commit-ID:  c5ae366e12b2bd56fc7d7e9d484836bec9ddc110
Gitweb:     http://git.kernel.org/tip/c5ae366e12b2bd56fc7d7e9d484836bec9ddc110
Author:     Daniel Axtens <dja@xxxxxxxxxx>
AuthorDate: Thu, 11 May 2017 06:11:39 +1000
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 20 Jun 2017 12:26:52 +0200

sched/fair: WARN() and refuse to set buddy when !se->on_rq

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:

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

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.

Just do it.

Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20170510201139.16236-1-dja@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 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 396bca9..cb3a3da 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6164,8 +6164,11 @@ 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)
@@ -6173,8 +6176,11 @@ 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)