Messages in this thread | | | From | Ram Muthiah <> | Date | Wed, 30 Oct 2019 15:50:00 -0700 | Subject | Re: NULL pointer dereference in pick_next_task_fair |
| |
On Tue, Oct 29, 2019 at 4:50 AM 'Quentin Perret' via kernel-team <kernel-team@android.com> wrote: > > 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 > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com. >
Quentin and I were able to create a setup which reproduces the issue.
Given this, I tried Peter's proposed fix and was still able to reproduce the issue unfortunately. Current patch is located here - https://android-review.googlesource.com/c/kernel/common/+/1153487
Our mitigation for this issue on the android-mainline branch has been to revert 67692435c411 ("sched: Rework pick_next_task() slow-path"). https://android-review.googlesource.com/c/kernel/common/+/1152564
I'll spend some time detailing repro steps next. I should be able to provide an update on those details early next week.
We appreciate the help so far. Thanks, Ram
| |