Re: [GIT PULL] io_uring fixes for 5.12-rc3

[Jens Axboe <axboe@kernel.dk>]

On 3/12/21 2:17 PM, Linus Torvalds wrote:
> On Fri, Mar 12, 2021 at 11:48 AM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> - Make IO threads unfreezable by default, on account of a bug report
>>   that had them spinning on resume. Honestly not quite sure why thawing
>>   leaves us with a perpetual signal pending (causing the spin), but for
>>   now make them unfreezable like there were in 5.11 and prior.
> 
> That "not quite sure" doesn't exactly give my the warm and fuzzies,
> but not being new is ok, I guess. But I'd really like you to try to
> figure out what is actually going on.

Oh I agree, which is why I wanted to bring it up as I do feel like we're
just working around this issue.

> I'm _guessing_ it's just that now those threads are user threads, and
> then the freezing logic expects them to freeze/thaw using a signal
> machinery or something like that. And that doesn't work when there is
> no signal handling for those threads.
> 
> But it would be good to _know_.

That's very much my guess too, since the symptom is that we loop all
the time because we think there's a signal pending. I am investigating
it and reached out to Rafael to try and figure this out, but at the same
time I did not want to have an -rc3 that caused resume issues for users.
Hence the "let's just disable freezing for now so that things work, get
it fixed for real when we can" kind of solution.

-- 
Jens Axboe