Re: NULL pointer dereference in pick_next_task_fair

Quentin Perret
2019-10-29T11:50:10Z

On Tuesday 29 Oct 2019 at 12:34:11 (+0100), Peter Zijlstra wrote:
> On Mon, Oct 28, 2019 at 05:46:03PM +0000, Quentin Perret wrote:
> > The issue is very transient and relatively hard to reproduce.
> >
> > After digging a bit, the offending commit seems to be:
> >
> > 67692435c411 ("sched: Rework pick_next_task() slow-path")
> >
> > By 'offending' I mean that reverting it makes the issue go away. The
> > issue comes from the fact that pick_next_entity() returns a NULL se in
> > the 'simple' path of pick_next_task_fair(), which causes obvious
> > problems in the subsequent call to set_next_entity().
> >
> > I'll dig more, but if anybody understands the issue in the meatime feel
> > free to send me a patch to try out :)
>
> Can you please see if this makes any difference?
>
> ---
> kernel/sched/core.c | 6 ++++--
> kernel/sched/fair.c | 2 +-
> kernel/sched/idle.c | 3 +--
> 3 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7880f4f64d0e..abd2d4f80381 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3922,8 +3922,10 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> goto restart;
>
> /* Assumes fair_sched_class->next == idle_sched_class */
> - if (unlikely(!p))
> - p = idle_sched_class.pick_next_task(rq, prev, rf);
> + if (unlikely(!p)) {
> + prev->sched_class->put_prev_task(rq, prev, rf);
> + p = idle_sched_class.pick_next_task(rq, NULL, NULL);
> + }
>
> return p;
> }
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 83ab35e2374f..2aad94bb7165 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6820,7 +6820,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
> simple:
> #endif
> if (prev)
> - put_prev_task(rq, prev);
> + prev->sched_class->put_prev_task(rq, prev, rf);
>
> do {
> se = pick_next_entity(cfs_rq, NULL);
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 8dad5aa600ea..e8dfc84f375a 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -390,8 +390,7 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
> {
> struct task_struct *next = rq->idle;
>
> - if (prev)
> - put_prev_task(rq, prev);
> + WARN_ON_ONCE(prev || rf);
>
> set_next_task_idle(rq, next);
>
>

Unfortunately the issue went into hiding this morning and I struggle to
reproduce it (this is turning bordeline nightmare now TBH).

I'll try the patch once my reproducer is fixed :/

Thank you very much for the help,
Quentin