Re: [Qemu-devel] [PATCH 10/10] blockjob: use deferred_to_main_loop to indicate the coroutine has ended

[John Snow <jsnow@redhat.com>]

On 03/23/2017 01:39 PM, Paolo Bonzini wrote:
> All block jobs are using block_job_defer_to_main_loop as the final
> step just before the coroutine terminates.  At this point,
> block_job_enter should do nothing, but currently it restarts
> the freed coroutine.
> 
> Now, the job->co states should probably be changed to an enum
> (e.g. BEFORE_START, STARTED, YIELDED, COMPLETED) subsuming

Yes, I'd love to formalize the FSM for jobs.

> block_job_started, job->deferred_to_main_loop and job->busy.
> For now, this patch eliminates the problematic reenter by
> removing the reset of job->deferred_to_main_loop (which served
> no purpose, as far as I could see) and checking the flag in
> block_job_enter.

Not sure -- the original commit 794f01414 makes it seem like it should
stay so that the correct AIO context can be acquired. Probably a race as
jobs don't often stay long once they've deferred to the main loop, but I
think the reset is harmless as you say.

> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  blockjob.c                   | 10 ++++++++--
>  include/block/blockjob_int.h |  3 ++-
>  2 files changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/blockjob.c b/blockjob.c
> index 3fa2885..2d80dae 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -750,7 +750,14 @@ void block_job_resume_all(void)
>  
>  void block_job_enter(BlockJob *job)
>  {
> -    if (job->co && !job->busy) {
> +    if (!block_job_started(job)) {
> +        return;
> +    }
> +    if (job->deferred_to_main_loop) {
> +        return;
> +    }
> +
> +    if (!job->busy) {
>          qemu_coroutine_enter(job->co);
>      }
>  }
> @@ -874,7 +881,6 @@ static void block_job_defer_to_main_loop_bh(void *opaque)
>          aio_context_acquire(aio_context);
>      }
>  
> -    data->job->deferred_to_main_loop = false;
>      data->fn(data->job, data->opaque);
>  
>      if (aio_context != data->aio_context) {
> diff --git a/include/block/blockjob_int.h b/include/block/blockjob_int.h
> index 97ffc43..4d287ba 100644
> --- a/include/block/blockjob_int.h
> +++ b/include/block/blockjob_int.h
> @@ -227,7 +227,8 @@ typedef void BlockJobDeferToMainLoopFn(BlockJob *job, void *opaque);
>   * @fn: The function to run in the main loop
>   * @opaque: The opaque value that is passed to @fn
>   *
> - * Execute a given function in the main loop with the BlockDriverState
> + * This function must be called by the main job coroutine just before it
> + * returns.  @fn is executed in the main loop with the BlockDriverState
>   * AioContext acquired.  Block jobs must call bdrv_unref(), bdrv_close(), and
>   * anything that uses bdrv_drain_all() in the main loop.
>   *
> 

Reviewed-by: John Snow <jsnow@redhat.com>