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 <[email protected]> Cc: Ben Segall <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Thomas Gleixner <[email protected]> Signed-off-by: Daniel Axtens <[email protected]> --- 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
