Re: [PATCH] sched: fix rq lock recursion issue

[Satya Durga Srinivasu Prabhala <quic_satyap@quicinc.com>]

On 6/30/22 2:53 PM, Qais Yousef wrote:
> Hi Satya
>
> On 06/24/22 00:42, Satya Durga Srinivasu Prabhala wrote:
>> Below recursion is observed in a rare scenario where __schedule()
>> takes rq lock, at around same time task's affinity is being changed,
>> bpf function for tracing sched_switch calls migrate_enabled(),
>> checks for affinity change (cpus_ptr != cpus_mask) lands into
>> __set_cpus_allowed_ptr which tries acquire rq lock and causing the
>> recursion bug.
>>
>> Fix the issue by switching to preempt_enable/disable() for non-RT
>> Kernels.
> Interesting bug. Thanks for the report. Unfortunately I can't see this being
> a fix as it just limits the bug visibility to PREEMPT_RT kernels, but won't fix
> anything, no? ie: Kernels compiled with PREEMPT_RT will still hit this failure.
Thanks for taking a look. That's right.
> I'm curious how the race with set affinity is happening. I would have thought
> user space would get blocked as __schedule() will hold the rq lock.
>
> Do you have more details on that?
I'm still trying to figure out how we even end with up such a race. No 
easy way repro
the issue, I'm checking internally with some debug logs.
>
> Thanks
>
> --
> Qais Yousef
>
>> -010 |spin_bug(lock = ???, msg = ???)
>> -011 |debug_spin_lock_before(inline)
>> -011 |do_raw_spin_lock(lock = 0xFFFFFF89323BB600)
>> -012 |_raw_spin_lock(inline)
>> -012 |raw_spin_rq_lock_nested(inline)
>> -012 |raw_spin_rq_lock(inline)
>> -012 |task_rq_lock(p = 0xFFFFFF88CFF1DA00, rf = 0xFFFFFFC03707BBE8)
>> -013 |__set_cpus_allowed_ptr(inline)
>> -013 |migrate_enable()
>> -014 |trace_call_bpf(call = ?, ctx = 0xFFFFFFFDEF954600)
>> -015 |perf_trace_run_bpf_submit(inline)
>> -015 |perf_trace_sched_switch(__data = 0xFFFFFFE82CF0BCB8, preempt = FALSE, prev = ?, next = ?)
>> -016 |__traceiter_sched_switch(inline)
>> -016 |trace_sched_switch(inline)
>> -016 |__schedule(sched_mode = ?)
>> -017 |schedule()
>> -018 |arch_local_save_flags(inline)
>> -018 |arch_irqs_disabled(inline)
>> -018 |__raw_spin_lock_irq(inline)
>> -018 |_raw_spin_lock_irq(inline)
>> -018 |worker_thread(__worker = 0xFFFFFF88CE251300)
>> -019 |kthread(_create = 0xFFFFFF88730A5A80)
>> -020 |ret_from_fork(asm)
>>
>> Signed-off-by: Satya Durga Srinivasu Prabhala <quic_satyap@quicinc.com>
>> ---
>>   kernel/sched/core.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index bfa7452ca92e..e254e9227341 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2223,6 +2223,7 @@ static void migrate_disable_switch(struct rq *rq, struct task_struct *p)
>>   
>>   void migrate_disable(void)
>>   {
>> +#ifdef CONFIG_PREEMPT_RT
>>   	struct task_struct *p = current;
>>   
>>   	if (p->migration_disabled) {
>> @@ -2234,11 +2235,15 @@ void migrate_disable(void)
>>   	this_rq()->nr_pinned++;
>>   	p->migration_disabled = 1;
>>   	preempt_enable();
>> +#else
>> +	preempt_disable();
>> +#endif
>>   }
>>   EXPORT_SYMBOL_GPL(migrate_disable);
>>   
>>   void migrate_enable(void)
>>   {
>> +#ifdef CONFIG_PREEMPT_RT
>>   	struct task_struct *p = current;
>>   
>>   	if (p->migration_disabled > 1) {
>> @@ -2265,6 +2270,9 @@ void migrate_enable(void)
>>   	p->migration_disabled = 0;
>>   	this_rq()->nr_pinned--;
>>   	preempt_enable();
>> +#else
>> +	preempt_enable();
>> +#endif
>>   }
>>   EXPORT_SYMBOL_GPL(migrate_enable);
>>   
>> -- 
>> 2.36.1
>>