Re: [Qemu-devel] [PATCH v2 03/11] Blockjobs: Internalize user_pause logic

[Jeff Cody <jcody@redhat.com>]

On Fri, Sep 30, 2016 at 06:00:41PM -0400, John Snow wrote:
> BlockJobs will begin hiding their state in preparation for some
> refactorings anyway, so let's internalize the user_pause mechanism
> instead of leaving it to callers to correctly manage.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  block/io.c               |  2 +-
>  blockdev.c               | 10 ++++------
>  blockjob.c               | 16 ++++++++++++----
>  include/block/blockjob.h | 11 ++++++++++-
>  4 files changed, 27 insertions(+), 12 deletions(-)
> 
> diff --git a/block/io.c b/block/io.c
> index fdf7080..868b065 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -291,7 +291,7 @@ void bdrv_drain_all(void)
>          AioContext *aio_context = blk_get_aio_context(job->blk);
>  
>          aio_context_acquire(aio_context);
> -        block_job_pause(job);
> +        block_job_pause(job, false);

Hmm.  Maybe semantically, an enum might work better here than a bool.  When
I see "block_job_pause(job, false)", it reads like you just un-paused the
job, rather than paused it with some contextual info.  Perhaps
JOB_USER_PAUSE and JOB_SYS_PAUSE (or just JOB_USER and JOB_SYS)?

>          aio_context_release(aio_context);
>      }
>  
> diff --git a/blockdev.c b/blockdev.c
> index 03200e7..268452f 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -3629,7 +3629,7 @@ void qmp_block_job_cancel(const char *device,
>          force = false;
>      }
>  
> -    if (job->user_paused && !force) {
> +    if (block_job_paused(job) && !force) {
>          error_setg(errp, "The block job for device '%s' is currently paused",
>                     device);
>          goto out;
> @@ -3646,13 +3646,12 @@ void qmp_block_job_pause(const char *device, Error **errp)
>      AioContext *aio_context;
>      BlockJob *job = find_block_job(device, &aio_context, errp);
>  
> -    if (!job || job->user_paused) {
> +    if (!job || block_job_paused(job)) {
>          return;
>      }
>  
> -    job->user_paused = true;
>      trace_qmp_block_job_pause(job);
> -    block_job_pause(job);
> +    block_job_pause(job, true);
>      aio_context_release(aio_context);
>  }
>  
> @@ -3661,11 +3660,10 @@ void qmp_block_job_resume(const char *device, Error **errp)
>      AioContext *aio_context;
>      BlockJob *job = find_block_job(device, &aio_context, errp);
>  
> -    if (!job || !job->user_paused) {
> +    if (!job || !block_job_paused(job)) {
>          return;
>      }
>  
> -    job->user_paused = false;
>      trace_qmp_block_job_resume(job);
>      block_job_iostatus_reset(job);
>      block_job_resume(job);
> diff --git a/blockjob.c b/blockjob.c
> index 6a300ba..2a35f50 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -104,7 +104,7 @@ static void block_job_detach_aio_context(void *opaque)
>      /* In case the job terminates during aio_poll()... */
>      block_job_ref(job);
>  
> -    block_job_pause(job);
> +    block_job_pause(job, false);
>  
>      if (!job->paused) {
>          /* If job is !job->busy this kicks it into the next pause point. */
> @@ -343,9 +343,12 @@ void block_job_complete(BlockJob *job, Error **errp)
>      job->driver->complete(job, errp);
>  }
>  
> -void block_job_pause(BlockJob *job)
> +void block_job_pause(BlockJob *job, bool user)
>  {
>      job->pause_count++;
> +    if (user) {
> +        job->user_paused = true;
> +    }
>  }
>  
>  static bool block_job_should_pause(BlockJob *job)
> @@ -353,6 +356,11 @@ static bool block_job_should_pause(BlockJob *job)
>      return job->pause_count > 0;
>  }
>  
> +bool block_job_paused(BlockJob *job)

I think a more apt name for this would be "block_job_user_paused()", because
it doesn't tell if the job is paused per se, but only if it is paused by a
user.

> +{
> +    return job ? job->user_paused : 0;
> +}
> +
>  void coroutine_fn block_job_pause_point(BlockJob *job)
>  {
>      if (!block_job_should_pause(job)) {
> @@ -386,6 +394,7 @@ void block_job_resume(BlockJob *job)
>      if (job->pause_count) {
>          return;
>      }
> +    job->user_paused = false;

At first I was thinking block_job_resume() would need a way of knowing if it
was a user resume, similar to block_job_pause().  But I guess not... if the
user paused it, then job->pause_count should always be > 0, I think.

But wait...could we ever get into a state where something like this happens:

block_job_pause(job, true);  /* user paused */

[...]

block_job_pause(job, false); /* system paused */

[...]

block_job_resume(job);       /* user resumes */

[...]   /* job->user_paused is still true, although it shouldn't be */

block_job_resume(job);       /* system resumes */

                             /* now job->user_paused is false */


>      block_job_enter(job);
>  }
>  
> @@ -592,8 +601,7 @@ BlockErrorAction block_job_error_action(BlockJob *job, BlockdevOnError on_err,
>                                      action, &error_abort);
>      if (action == BLOCK_ERROR_ACTION_STOP) {
>          /* make the pause user visible, which will be resumed from QMP. */
> -        job->user_paused = true;
> -        block_job_pause(job);
> +        block_job_pause(job, true);
>          block_job_iostatus_set_err(job, error);
>      }
>      return action;
> diff --git a/include/block/blockjob.h b/include/block/blockjob.h
> index 4ddb4ae..081f6c2 100644
> --- a/include/block/blockjob.h
> +++ b/include/block/blockjob.h
> @@ -347,10 +347,19 @@ void coroutine_fn block_job_pause_point(BlockJob *job);
>  /**
>   * block_job_pause:
>   * @job: The job to be paused.
> + * @user: Requested explicitly via user?
>   *
>   * Asynchronously pause the specified job.
>   */
> -void block_job_pause(BlockJob *job);
> +void block_job_pause(BlockJob *job, bool user);
> +
> +/**
> + * block_job_paused:
> + * @job: The job to query.
> + *
> + * Returns true if the job is user-paused.
> + */
> +bool block_job_paused(BlockJob *job);
>  
>  /**
>   * block_job_resume:
> -- 
> 2.7.4
>