Re: [PATCH 1/2] sched: Deny self-issued __set_cpus_allowed_ptr() when migrate_disable()

From: Valentin Schneider
Date: Thu Oct 15 2020 - 05:14:35 EST



On 13/10/20 15:21, Peter Zijlstra wrote:
> On Tue, Oct 13, 2020 at 04:15:08PM +0200, Sebastian Andrzej Siewior wrote:
>> On 2020-10-13 15:01:15 [+0100], Valentin Schneider wrote:
>> > migrate_disable();
>> > set_cpus_allowed_ptr(current, {something excluding task_cpu(current)});
>> > affine_move_task(); <-- never returns
>> >
>> > Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
>> > ---
>> > kernel/sched/core.c | 5 +++++
>> > 1 file changed, 5 insertions(+)
>> >
>> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> > index 4ccd1099adaa..7f4e38819de1 100644
>> > --- a/kernel/sched/core.c
>> > +++ b/kernel/sched/core.c
>> > @@ -2189,6 +2189,11 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>> > if (!(flags & SCA_MIGRATE_ENABLE) && cpumask_equal(&p->cpus_mask, new_mask))
>> > goto out;
>> >
>> > + if (p == current &&
>> > + is_migration_disabled(p) &&
>> > + !cpumask_test_cpu(task_cpu(p), new_mask))
>> > + ret = -EBUSY;
>> > +
>>
>> This shouldn't happen, right? The function may sleep so it shouldn't be
>> entered with disabled migration. A WARN_ON might spot the bad caller.
>
> So yeah, I like detecting the case but agree with bigeasy that an
> additional WARN would make sense, lemme go add that.

Err, I've just realized that this will warn on migrate_enable() if there
are pending affinity changes, since p->migration_disabled is cleared
*after* the call to __set_cpus_allowed_ptr().

This wants:
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d503d6cb8350..e8156e3d3b4a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2266,7 +2266,8 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
if (!(flags & SCA_MIGRATE_ENABLE) && cpumask_equal(&p->cpus_mask, new_mask))
goto out;

- if (WARN_ON_ONCE(p == current &&
+ if (WARN_ON_ONCE(!(flags & SCA_MIGRATE_ENABLE) &&
+ p == current &&
is_migration_disabled(p) &&
!cpumask_test_cpu(task_cpu(p), new_mask)))
ret = -EBUSY;
---