Qian reported that some fuzzer issuing sched_setaffinity() ends up stuck on
a wait_for_completion(). The problematic pattern seems to be:
affine_move_task()
// task_running() case
stop_one_cpu();
wait_for_completion(&pending->done);
Combined with, on the stopper side:
migration_cpu_stop()
// Task moved between unlocks and scheduling the stopper
task_rq(p) != rq &&
// task_running() case
dest_cpu >= 0
=> no complete_all()
This can happen with both PREEMPT and !PREEMPT, although !PREEMPT should
be more likely to see this given the targeted task has a much bigger window
to block and be woken up elsewhere before the stopper runs.
Make migration_cpu_stop() always look at pending affinity requests; signal
their completion if the stopper hits a rq mismatch but the task is
still within its allowed mask. When Migrate-Disable isn't involved, this
matches the previous set_cpus_allowed_ptr() vs migration_cpu_stop()
behaviour.
Link: https://lore.kernel.org/lkml/[email protected]
Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Reported-by: Qian Cai <[email protected]>
Signed-off-by: Valentin Schneider <[email protected]>
---
kernel/sched/core.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 02076e6d3792..fad0a8e62aca 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1923,7 +1923,7 @@ static int migration_cpu_stop(void *data)
else
p->wake_cpu = dest_cpu;
- } else if (dest_cpu < 0) {
+ } else if (dest_cpu < 0 || pending) {
/*
* This happens when we get migrated between migrate_enable()'s
* preempt_enable() and scheduling the stopper task. At that
@@ -1933,6 +1933,17 @@ static int migration_cpu_stop(void *data)
* more likely.
*/
+ /*
+ * The task moved before the stopper got to run. We're holding
+ * ->pi_lock, so the allowed mask is stable - if it got
+ * somewhere allowed, we're done.
+ */
+ if (pending && cpumask_test_cpu(task_cpu(p), p->cpus_ptr)) {
+ p->migration_pending = NULL;
+ complete = true;
+ goto out;
+ }
+
/*
* When this was migrate_enable() but we no longer have an
* @pending, a concurrent SCA 'fixed' things and we should be
--
2.27.0
Hi,
On Fri, Nov 13, 2020 at 11:24:14AM +0000, Valentin Schneider wrote:
> Qian reported that some fuzzer issuing sched_setaffinity() ends up stuck on
> a wait_for_completion(). The problematic pattern seems to be:
> affine_move_task()
> // task_running() case
> stop_one_cpu();
> wait_for_completion(&pending->done);
>
> Combined with, on the stopper side:
>
> migration_cpu_stop()
> // Task moved between unlocks and scheduling the stopper
> task_rq(p) != rq &&
> // task_running() case
> dest_cpu >= 0
>
> => no complete_all()
>
> This can happen with both PREEMPT and !PREEMPT, although !PREEMPT should
> be more likely to see this given the targeted task has a much bigger window
> to block and be woken up elsewhere before the stopper runs.
>
> Make migration_cpu_stop() always look at pending affinity requests; signal
> their completion if the stopper hits a rq mismatch but the task is
> still within its allowed mask. When Migrate-Disable isn't involved, this
> matches the previous set_cpus_allowed_ptr() vs migration_cpu_stop()
> behaviour.
>
> Link: https://lore.kernel.org/lkml/[email protected]
> Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
> Reported-by: Qian Cai <[email protected]>
> Signed-off-by: Valentin Schneider <[email protected]>
> ---
> kernel/sched/core.c | 13 ++++++++++++-
> 1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 02076e6d3792..fad0a8e62aca 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1923,7 +1923,7 @@ static int migration_cpu_stop(void *data)
> else
> p->wake_cpu = dest_cpu;
>
> - } else if (dest_cpu < 0) {
> + } else if (dest_cpu < 0 || pending) {
> /*
> * This happens when we get migrated between migrate_enable()'s
> * preempt_enable() and scheduling the stopper task. At that
> @@ -1933,6 +1933,17 @@ static int migration_cpu_stop(void *data)
> * more likely.
> */
>
> + /*
> + * The task moved before the stopper got to run. We're holding
> + * ->pi_lock, so the allowed mask is stable - if it got
> + * somewhere allowed, we're done.
> + */
> + if (pending && cpumask_test_cpu(task_cpu(p), p->cpus_ptr)) {
> + p->migration_pending = NULL;
> + complete = true;
> + goto out;
> + }
> +
> /*
> * When this was migrate_enable() but we no longer have an
> * @pending, a concurrent SCA 'fixed' things and we should be
> --
> 2.27.0
Oh, I did not receive this patch from '[email protected]'
account. Checked that you sent the patch to that mail address
from web. If 'ouwen210' is not a good mail account name(I have
used this name since 2002), I will change to use this one(Now
is smooth enough and can go to lkml).
Thanks,
Tao
On 15/11/20 11:59, Tao Zhou wrote:
> Oh, I did not receive this patch from '[email protected]'
> account. Checked that you sent the patch to that mail address
> from web. If 'ouwen210' is not a good mail account name(I have
> used this name since 2002), I will change to use this one(Now
> is smooth enough and can go to lkml).
>
Nah, any address is fine. I grabbed whatever emails were cc'd to the patch
this is fixing - just make sure you're using the right address (i.e. the
one you want LKML stuff to go to) when interacting on the list and you
should be good.
> Thanks,
> Tao