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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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