linux-kernel - Re: [Intel-gfx] signal: break out of wait loops on kthread

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Message-ID: <87a64brlao.fsf@email.froward.int.ebiederm.org>
Date:   Mon, 28 Nov 2022 12:22:39 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        "Intel-gfx@...ts.freedesktop.org" <Intel-gfx@...ts.freedesktop.org>,
        linux-kernel@...r.kernel.org, sultan@...neltoast.com
Subject: Re: [Intel-gfx] signal: break out of wait loops on kthread_stop()

Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com> writes:

> On 19/10/2022 21:19, Jason A. Donenfeld wrote:
>> On Wed, Oct 19, 2022 at 09:09:28PM +0100, Tvrtko Ursulin wrote:
>>> Hm why is kthread_stop() after kthread_run() abuse? I don't see it in
>>> kerneldoc that it must not be used for stopping threads.
>> Because you don't want it to stop. You want to wait until it's done. If
>> you call stop right after run, it will even stop it before it even
>> begins to run. That's why you wind up sprinkling your msleeps
>> everywhere, indicating that clearly this is not meant to work that way.
> Not after kthread_run which wakes it up already. If the kerneldoc for
> kthread_stop() is correct at least... In which case I really do think 
> that the yields are pointless/red herring. Perhaps they predate kthread_run and
> then they were even wrong.
>
>>> Yep the yields and sleeps are horrible and will go. But they are also
>>> not relevant for the topic at hand.
>> Except they very much are. The reason you need these is because you're
>> using kthread_stop() for something it's not meant to do.
>
> It is supposed to assert kthread_should_stop() which thread can look at as when
> to exit. Except that now it can fail to get to that controlled exit
> point. Granted that argument is moot since it implies incomplete error handling
> in the thread anyway.
>
> Btw there are actually two use cases in our code base. One is thread controls
> the exit, second is caller controls the exit. Anyway...
>
>>> Never mind, I was not looking for anything more than a suggestion on how
>>> to maybe work around it in piece as someone is dealing with the affected
>>> call sites.
>> Sultan's kthread_work idea is probably the right direction. This would
>> seem to have what you need.
>
> ... yes, it can be converted. Even though for one of the two use cases we need
> explicit signalling. There now isn't anything which would assert
> kthread_should_stop() without also asserting the signal, right?. Neither 
> I found that the thread work API can do it.
>
> Fingers crossed we were the only "abusers" of the API. There's a quite a number
> of kthread_stop callers and it would be a large job to audit them all.


I have been out and am coming to this late.   Did this get resolved?


I really don't expect this affected much of anything else as the code
sat in linux-next for an entire development cycle before being merged.

But I would like to make certain problems with this change were resolved.

Thank you,
Eric