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Re: [Qemu-block] [PATCH 02/21] jobs: add exit shim

From: John Snow
Subject: Re: [Qemu-block] [PATCH 02/21] jobs: add exit shim
Date: Wed, 8 Aug 2018 11:38:04 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0

On 08/08/2018 11:23 AM, Kevin Wolf wrote:
> Am 08.08.2018 um 06:02 hat Jeff Cody geschrieben:
>> On Tue, Aug 07, 2018 at 12:33:30AM -0400, John Snow wrote:
>>> Most jobs do the same thing when they leave their running loop:
>>> - Store the return code in a structure
>>> - wait to receive this structure in the main thread
>>> - signal job completion via job_completed
>>> More seriously, when we utilize job_defer_to_main_loop_bh to call
>>> a function that calls job_completed, job_finalize_single will run
>>> in a context where it has recursively taken the aio_context lock,
>>> which can cause hangs if it puts down a reference that causes a flush.
>>> The job infrastructure is perfectly capable of registering job
>>> completion itself when we leave the job's entry point. In this
>>> context, we can signal job completion from outside of the aio_context,
>>> which should allow for job cleanup code to run with only one lock.
>>> Signed-off-by: John Snow <address@hidden>
>> I like the simplification, both in SLOC and in exit logic (as seen in
>> patches 3-7).
> I agree, unifying this seems like a good idea.
> Like in the first patch, I'm not convinced of the details, though.
> Essentially, this is my objection regarding job->err extended to
> job->ret: You rely on jobs setting job->ret and job->err, but the
> interfaces don't really show this.
>>> @@ -546,6 +559,12 @@ static void coroutine_fn job_co_entry(void *opaque)
>>>      assert(job && job->driver && job->driver->start);
>>>      job_pause_point(job);
>>>      job->driver->start(job);
>> One nit-picky observation here, that is unrelated to this patch: reading
>> through, it may not be so obvious that 'start' is really a 'run' or
>> 'execute', (linguistically, to me 'start' implies a kick-off rather than
>> ongoing execution).
> I had exactly the same thought. My proposal is to change the existing...
>     CoroutineEntry *start;
> ...which is just short for...
>     void coroutine_fn start(void *opaque);
> ...into this one:
>     int coroutine_fn run(void *opaque, Error **errp);
> I see that at the end of the series, you actually introduced an int
> return value already. I would have done that from the start, but as long
> the final state makes sense, I won't insist.
> But can we have the Error **errp addition, too? Pretty please?
> Kevin

I'm actually glad you want that addition, I was considering very
strongly adding it but I felt like I had made the series long enough
already and didn't want to change too much all at once.

The basic thought was just:

"It'd sure be nice to have a generic function entry point that looks
like it returns the same error information as our non-coroutine functions."

I can absolutely work that in, and break this series into two parts:

(1) Rework jobs infrastructure to use the new run signature, and
(2) Rework jobs to use the finalization callbacks.

Sound good?


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