> On Jul 6, 2022, at 2:40 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Wed, 6 Jul 2022 21:37:52 +0000
> Song Liu <songliubraving@xxxxxx> wrote:
>
>>> Can you comment here that returning -EAGAIN will not cause this to repeat.
>>> That it will change things where the next try will not return -EGAIN?
>>
>> Hmm.. this is not the guarantee here. This conflict is a real race condition
>> that an IPMODIFY function (i.e. livepatch) is being registered at the same time
>> when something else, for example bpftrace, is updating the BPF trampoline.
>>
>> This EAGAIN will propagate to the user of the IPMODIFY function (i.e. livepatch),
>> and we need to retry there. In the case of livepatch, the retry is initiated
>> from user space.
>
> We need to be careful here then. If there's a userspace application that
> runs at real-time and does a:
>
> do {
> errno = 0;
> regsiter_bpf();
> } while (errno != -EAGAIN);
Actually, do you mean:
do {
errno = 0;
regsiter_bpf();
} while (errno == -EAGAIN);
(== -EAGAIN) here?
In this specific race condition, register_bpf() will succeed, as it already
got tr->mutex. But the IPMODIFY (livepatch) side will fail and retry.
Since both livepatch and bpf trampoline changes are rare operations, I think
the chance of the race condition is low enough.
Does this make sense?
Thanks,
Song
