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

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

On 6/13/22 2:49 PM, Alexei Starovoitov wrote:
> On Mon, Jun 13, 2022 at 2:35 PM Satya Durga Srinivasu Prabhala
> <quic_satyap@quicinc.com> wrote:
>>
>> On 6/13/22 2:01 PM, Alexei Starovoitov wrote:
>>> is doesn't solve anything.
>>> Please provide a reproducer.
>> I'm trying to find an easy way to repro the issue, so far, unsuccessful.
>>
>>> iirc the task's affinity change can race even with preemption disabled
>>> on this cpu. Why would s/migrate/preemption/ address the deadlock ?
>> I don't think task's affinity change races with preemption disabled/enabled.
>>
>> Switching to preemption disable/enable calls helps as it's just simple
>> counter increment and decrement with a barrier, but with migrate
>> disable/enable when task's affinity changes, we run into recursive bug
>> due to rq lock.
> As Yonghong already explained, replacing migrate_disable
> with preempt_disable around bpf prog invocation is not an option.

If I understand correctly, Yonghong mentioned that replacing migrate_
with preempt_ won't work for RT Kernels and migrate_ APIs were introduced
for RT Kernels is what he was pointing to. I went back and cross checked
on 5.10 LTS Kernel, I see that the migrate_ calls end up just calling into
preemt_ calls [1]. So far non-RT kernels, sticking to preemt_ calls should
still work. Please let me know if I missed anything.

[1]
https://android.googlesource.com/kernel/common/+/refs/heads/android12-5.10/include/linux/preempt.h#335