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

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Date:   Tue, 13 Oct 2020 15:22:09 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
        qais.yousef@....com, swood@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vincent.donnefort@....com,
        tj@...nel.org
Subject: Re: [PATCH 1/2] sched: Deny self-issued __set_cpus_allowed_ptr() when migrate_disable()


On 13/10/20 15:15, 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@....com>
>> ---
>>  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.
>

Yeah, this is one of those paranoia-driven checks.

> Sebastian