Commit 00359a71 authored by John Snow's avatar John Snow Committed by Max Reitz
Browse files

jobs: add exit shim 



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: default avatarJohn Snow <jsnow@redhat.com>
Reviewed-by: default avatarMax Reitz <mreitz@redhat.com>
Message-id: 20180830015734.19765-4-jsnow@redhat.com
Reviewed-by: default avatarJeff Cody <jcody@redhat.com>
Signed-off-by: default avatarMax Reitz <mreitz@redhat.com>
parent 3d1f8b07

include/qemu/job.h

+11 −0
Original line number Diff line number Diff line
@@ -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

job.c

+18 −0
Original line number Diff line number Diff line
@@ -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);

    }

}