blockjob: Allow nested pause

This patch changes block_job_pause to increase the pause counter and
block_job_resume to decrease it.

The counter will allow calling block_job_pause/block_job_resume
unconditionally on a job when we need to suspend the IO temporarily.

From now on, each block_job_resume must be paired with a block_job_pause
to keep the counter balanced.

The user pause from QMP or HMP will only trigger block_job_pause once
until it's resumed, this is achieved by adding a user_paused flag in
BlockJob.

One occurrence of block_job_resume in mirror_complete is replaced with
block_job_enter which does what is necessary.

In block_job_cancel, the cancel flag is good enough to instruct
coroutines to quit loop, so use block_job_enter to replace the unpaired
block_job_resume.

Upon block job IO error, user is notified about the entering to the
pause state, so this pause belongs to user pause, set the flag
accordingly and expect a matching QMP resume.

[Extended doc comments as suggested by Paolo Bonzini
<pbonzini@redhat.com>.
--Stefan]

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 1428069921-2957-2-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 18 insertions, 4 deletions

diff --git a/include/block/blockjob.h b/include/block/blockjob.h
index b6d4ebbe03..57d8ef13e2 100644
--- a/include/block/blockjob.h
+++ b/include/block/blockjob.h
@@ -79,10 +79,16 @@ struct BlockJob {
     bool cancelled;
 
     /**
-     * Set to true if the job is either paused, or will pause itself
-     * as soon as possible (if busy == true).
+     * Counter for pause request. If non-zero, the block job is either paused,
+     * or if busy == true will pause itself as soon as possible.
      */
-    bool paused;
+    int pause_count;
+
+    /**
+     * Set to true if the job is paused by user.  Can be unpaused with the
+     * block-job-resume QMP command.
+     */
+    bool user_paused;
 
     /**
      * Set to false by the job while it is in a quiescent state, where
@@ -225,11 +231,19 @@ void block_job_pause(BlockJob *job);
  * block_job_resume:
  * @job: The job to be resumed.
  *
- * Resume the specified job.
+ * Resume the specified job.  Must be paired with a preceding block_job_pause.
  */
 void block_job_resume(BlockJob *job);
 
 /**
+ * block_job_enter:
+ * @job: The job to enter.
+ *
+ * Continue the specified job by entering the coroutine.
+ */
+void block_job_enter(BlockJob *job);
+
+/**
  * block_job_event_cancelled:
  * @job: The job whose information is requested.
  *