Re: [Qemu-block] [PATCH v2 1/3] blockjob: add block_job_start_shim

[Jeff Cody]

On Thu, Mar 16, 2017 at 05:23:49PM -0400, John Snow wrote:
> The purpose of this shim is to allow us to pause pre-started jobs.
> The purpose of *that* is to allow us to buffer a pause request that
> will be able to take effect before the job ever does any work, allowing
> us to create jobs during a quiescent state (under which they will be
> automatically paused), then resuming the jobs after the critical section
> in any order, either:
> 
> (1) -block_job_start
>     -block_job_resume (via e.g. drained_end)
> 
> (2) -block_job_resume (via e.g. drained_end)
>     -block_job_start
> 
> The problem that requires a startup wrapper is the idea that a job must
> start in the busy=true state only its first time-- all subsequent entries
> require busy to be false, and the toggling of this state is otherwise
> handled during existing pause and yield points.
> 
> The wrapper simply allows us to mandate that a job can "start," set busy
> to true, then immediately pause only if necessary. We could avoid
> requiring a wrapper, but all jobs would need to do it, so it's been
> factored out here.

I think this makes sense.  So when this happens:

* block_job_create
* block_job_pause
* block_job_resume  <-- only effects pause_count, rest is noop
* block_job_start

The block_job_resume is mostly a no-op, only affecting the pause_count but
since there is no job coroutine created yet, the block_job_enter does
nothing.

> 
> Signed-off-by: John Snow <address@hidden>
> ---
>  blockjob.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/blockjob.c b/blockjob.c
> index 69126af..69b4ec6 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -250,16 +250,28 @@ static bool block_job_started(BlockJob *job)
>      return job->co;
>  }
>  
> +/**
> + * All jobs must allow a pause point before entering their job proper. This
> + * ensures that jobs can be paused prior to being started, then resumed 
> later.
> + */
> +static void coroutine_fn block_job_co_entry(void *opaque)
> +{
> +    BlockJob *job = opaque;
> +
> +    assert(job && job->driver && job->driver->start);
> +    block_job_pause_point(job);
> +    job->driver->start(job);
> +}
> +
>  void block_job_start(BlockJob *job)
>  {
>      assert(job && !block_job_started(job) && job->paused &&
> -           !job->busy && job->driver->start);
> -    job->co = qemu_coroutine_create(job->driver->start, job);
> -    if (--job->pause_count == 0) {
> -        job->paused = false;
> -        job->busy = true;
> -        qemu_coroutine_enter(job->co);
> -    }
> +           job->driver && job->driver->start);
> +    job->co = qemu_coroutine_create(block_job_co_entry, job);
> +    job->pause_count--;
> +    job->busy = true;
> +    job->paused = false;
> +    qemu_coroutine_enter(job->co);
>  }
>  
>  void block_job_ref(BlockJob *job)
> -- 
> 2.9.3
>