Re: [Qemu-devel] [PATCH 07/11] blockjob: introduce block_job_cancel_async, check iostatus invariants

[John Snow]

On 04/19/2017 10:42 AM, Paolo Bonzini wrote:
> The new functions helps respecting the invariant that the coroutine
> is entered with false user_resume, zero pause count and no error
> recorded in the iostatus.
> 
> Resetting the iostatus is now common to all of block_job_cancel_async,
> block_job_user_resume and block_job_iostatus_reset, albeit with slight
> differences:
> 
> - block_job_cancel_async resets the iostatus, and resumes the job if
> there was an error, but the coroutine is not restarted immediately.
> For example the caller may continue with a call to block_job_finish_sync.
> 
> - block_job_user_resume resets the iostatus.  It wants to resume the job
> unconditionally, even if there was no error.
> 
> - block_job_iostatus_reset doesn't resume the job at all.  Maybe that's
> a bug but it should be fixed separately.
> 
> block_job_iostatus_reset does the least common denominator, so add some
> checking but otherwise leave it as the entry point for resetting the
> iostatus.
> 
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>         v1->v2: rewritten
> 
>  blockjob.c | 24 ++++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/blockjob.c b/blockjob.c
> index 5906266..1756153 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -304,6 +304,19 @@ static void block_job_completed_single(BlockJob *job)
>      block_job_unref(job);
>  }
>  
> +static void block_job_cancel_async(BlockJob *job)
> +{
> +    if (job->iostatus != BLOCK_DEVICE_IO_STATUS_OK) {
> +        block_job_iostatus_reset(job);
> +    }
> +    if (job->user_paused) {
> +        /* Do not call block_job_enter here, the caller will handle it.  */
> +        job->user_paused = false;
> +        job->pause_count--;
> +    }
> +    job->cancelled = true;
> +}
> +
>  static void block_job_completed_txn_abort(BlockJob *job)
>  {
>      AioContext *ctx;
> @@ -328,7 +341,7 @@ static void block_job_completed_txn_abort(BlockJob *job)
>               * them; this job, however, may or may not be cancelled, 
> depending
>               * on the caller, so leave it. */
>              if (other_job != job) {
> -                other_job->cancelled = true;
> +                block_job_cancel_async(other_job);
>              }
>              continue;
>          }
> @@ -411,8 +424,8 @@ bool block_job_user_paused(BlockJob *job)
>  void block_job_user_resume(BlockJob *job)
>  {
>      if (job && job->user_paused && job->pause_count > 0) {
> -        job->user_paused = false;
>          block_job_iostatus_reset(job);
> +        job->user_paused = false;
>          block_job_resume(job);
>      }
>  }
> @@ -420,8 +433,7 @@ void block_job_user_resume(BlockJob *job)
>  void block_job_cancel(BlockJob *job)
>  {
>      if (block_job_started(job)) {
> -        job->cancelled = true;
> -        block_job_iostatus_reset(job);
> +        block_job_cancel_async(job);
>          block_job_enter(job);>      } else {
>          block_job_completed(job, -ECANCELED);
> @@ -765,6 +777,10 @@ void block_job_yield(BlockJob *job)
>  
>  void block_job_iostatus_reset(BlockJob *job)
>  {
> +    if (job->iostatus == BLOCK_DEVICE_IO_STATUS_OK) {
> +        return;
> +    }
> +    assert(job->user_paused && job->pause_count > 0);

Why assert that it's user-paused? Will that be true from:

(A) All users of block_job_cancel_async, including:

- block_job_cancel
- block_job_completed
    block_job_completed_txn_abort

(B) all users of blk_iostatus_reset:
- blk_do_attach_dev
- qmp_cont

It's ... not really clear to me that this is true, can you help me out?

>      job->iostatus = BLOCK_DEVICE_IO_STATUS_OK;
>  }
>  
>