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Re: [Qemu-devel] [PATCH v3 3/9] jobs: add exit shim
From: |
Jeff Cody |
Subject: |
Re: [Qemu-devel] [PATCH v3 3/9] jobs: add exit shim |
Date: |
Fri, 31 Aug 2018 09:48:28 -0400 |
User-agent: |
Mutt/1.5.24 (2015-08-30) |
On Wed, Aug 29, 2018 at 09:57:28PM -0400, John Snow wrote:
> All 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
>
> Few jobs do anything beyond exactly this. Consolidate this exit
> logic for a net reduction in SLOC.
>
> 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.
>
> You can observe this in practice by looking at mirror_exit's careful
> placement of job_completed and bdrv_unref calls.
>
> If we centralize job exiting, we can signal job completion from outside
> of the aio_context, which should allow for job cleanup code to run with
> only one lock, which makes cleanup callbacks less tricky to write.
>
> Signed-off-by: John Snow <address@hidden>
> Reviewed-by: Max Reitz <address@hidden>
Reviewed-by: Jeff Cody <address@hidden>
> ---
> include/qemu/job.h | 11 +++++++++++
> job.c | 18 ++++++++++++++++++
> 2 files changed, 29 insertions(+)
>
> diff --git a/include/qemu/job.h b/include/qemu/job.h
> index e0e99870a1..1144d671a1 100644
> --- a/include/qemu/job.h
> +++ b/include/qemu/job.h
> @@ -208,6 +208,17 @@ struct JobDriver {
> */
> void (*drain)(Job *job);
>
> + /**
> + * If the callback is not NULL, exit will be invoked from the main thread
> + * when the job's coroutine has finished, but before transactional
> + * convergence; before @prepare or @abort.
> + *
> + * FIXME TODO: This callback is only temporary to transition remaining
> jobs
> + * to prepare/commit/abort/clean callbacks and will be removed before
> 3.1.
> + * is released.
> + */
> + void (*exit)(Job *job);
> +
> /**
> * If the callback is not NULL, prepare will be invoked when all the jobs
> * belonging to the same transaction complete; or upon this job's
> completion
> diff --git a/job.c b/job.c
> index bc1d970df4..bc8dad4e71 100644
> --- a/job.c
> +++ b/job.c
> @@ -535,6 +535,18 @@ void job_drain(Job *job)
> }
> }
>
> +static void job_exit(void *opaque)
> +{
> + Job *job = (Job *)opaque;
> + AioContext *aio_context = job->aio_context;
> +
> + if (job->driver->exit) {
> + aio_context_acquire(aio_context);
> + job->driver->exit(job);
> + aio_context_release(aio_context);
> + }
> + job_completed(job, job->ret);
> +}
>
> /**
> * All jobs must allow a pause point before entering their job proper. This
> @@ -547,6 +559,12 @@ static void coroutine_fn job_co_entry(void *opaque)
> assert(job && job->driver && job->driver->run);
> job_pause_point(job);
> job->ret = job->driver->run(job, &job->err);
> + if (!job->deferred_to_main_loop) {
> + job->deferred_to_main_loop = true;
> + aio_bh_schedule_oneshot(qemu_get_aio_context(),
> + job_exit,
> + job);
> + }
> }
>
>
> --
> 2.14.4
>
- Re: [Qemu-devel] [PATCH v3 5/9] block/mirror: utilize job_exit shim, (continued)
- [Qemu-devel] [PATCH v3 7/9] block/backup: make function variables consistently named, John Snow, 2018/08/29
- [Qemu-devel] [PATCH v3 8/9] jobs: remove ret argument to job_completed; privatize it, John Snow, 2018/08/29
- [Qemu-devel] [PATCH v3 9/9] jobs: remove job_defer_to_main_loop, John Snow, 2018/08/29
- [Qemu-devel] [PATCH v3 4/9] block/commit: utilize job_exit shim, John Snow, 2018/08/29
- [Qemu-devel] [PATCH v3 6/9] jobs: utilize job_exit shim, John Snow, 2018/08/29
- [Qemu-devel] [PATCH v3 3/9] jobs: add exit shim, John Snow, 2018/08/29
- Re: [Qemu-devel] [PATCH v3 3/9] jobs: add exit shim,
Jeff Cody <=
- Re: [Qemu-devel] [PATCH v3 0/9] jobs: Job Exit Refactoring Pt 1, Max Reitz, 2018/08/31