blockjob: add pause points

Block jobs are coroutines that usually perform I/O but sometimes also
sleep or yield.  Currently only sleeping or yielded block jobs can be
paused.  This means jobs that do not sleep or yield (using
block_job_yield()) are unaffected by block_job_pause().

Add block_job_pause_point() so that block jobs can mark quiescent points
that are suitable for pausing.  This solves the problem that it can take
a block job a long time to pause if it is performing a long series of
I/O operations.

Transitioning to paused state involves a .pause()/.resume() callback.
These callbacks are used to ensure that I/O and event loop activity has
ceased while the job is at a pause point.

Note that this patch introduces a stricter pause state than previously.
The job->busy flag was incorrectly documented as a quiescent state
without I/O pending.  This is violated by any job that has I/O pending
across sleep or block_job_yield(), like the mirror block job.

[Add missing block_job_should_pause() check to avoid deadlock after
job->driver->pause() in block_job_pause_point().
--Stefan]

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1466096189-6477-4-git-send-email-stefanha@redhat.com

blockjob.c

@@ -257,6 +257,32 @@ static bool block_job_should_pause(BlockJob *job)
     return job->pause_count > 0;
 }
 
+void coroutine_fn block_job_pause_point(BlockJob *job)
+{
+    if (!block_job_should_pause(job)) {
+        return;
+    }
+    if (block_job_is_cancelled(job)) {
+        return;
+    }
+
+    if (job->driver->pause) {
+        job->driver->pause(job);
+    }
+
+    if (block_job_should_pause(job) && !block_job_is_cancelled(job)) {
+        job->paused = true;
+        job->busy = false;
+        qemu_coroutine_yield(); /* wait for block_job_resume() */
+        job->busy = true;
+        job->paused = false;
+    }
+
+    if (job->driver->resume) {
+        job->driver->resume(job);
+    }
+}
+
 void block_job_resume(BlockJob *job)
 {
     assert(job->pause_count > 0);
@@ -364,11 +390,9 @@ void block_job_sleep_ns(BlockJob *job, QEMUClockType type, int64_t ns)
     if (!block_job_should_pause(job)) {
         co_aio_sleep_ns(blk_get_aio_context(job->blk), type, ns);
     }
-    /* The job can be paused while sleeping, so check this again */
-    if (block_job_should_pause(job)) {
-        qemu_coroutine_yield();
-    }
     job->busy = true;
+
+    block_job_pause_point(job);
 }
 
 void block_job_yield(BlockJob *job)
@@ -381,8 +405,12 @@ void block_job_yield(BlockJob *job)
     }
 
     job->busy = false;
-    qemu_coroutine_yield();
+    if (!block_job_should_pause(job)) {
+        qemu_coroutine_yield();
+    }
     job->busy = true;
+
+    block_job_pause_point(job);
 }
 
 BlockJobInfo *block_job_query(BlockJob *job)

include/block/blockjob.h

@@ -70,6 +70,20 @@ typedef struct BlockJobDriver {
      * never both.
      */
     void (*abort)(BlockJob *job);
+
+    /**
+     * If the callback is not NULL, it will be invoked when the job transitions
+     * into the paused state.  Paused jobs must not perform any asynchronous
+     * I/O or event loop activity.  This callback is used to quiesce jobs.
+     */
+    void coroutine_fn (*pause)(BlockJob *job);
+
+    /**
+     * If the callback is not NULL, it will be invoked when the job transitions
+     * out of the paused state.  Any asynchronous I/O or event loop activity
+     * should be restarted from this callback.
+     */
+    void coroutine_fn (*resume)(BlockJob *job);
 } BlockJobDriver;
 
 /**
@@ -119,12 +133,18 @@ struct BlockJob {
     bool user_paused;
 
     /**
-     * Set to false by the job while it is in a quiescent state, where
-     * no I/O is pending and the job has yielded on any condition
-     * that is not detected by #aio_poll, such as a timer.
+     * Set to false by the job while the coroutine has yielded and may be
+     * re-entered by block_job_enter().  There may still be I/O or event loop
+     * activity pending.
      */
     bool busy;
 
+    /**
+     * Set to true by the job while it is in a quiescent state, where
+     * no I/O or event loop activity is pending.
+     */
+    bool paused;
+
     /**
      * Set to true when the job is ready to be completed.
      */
@@ -298,6 +318,15 @@ bool block_job_is_cancelled(BlockJob *job);
  */
 BlockJobInfo *block_job_query(BlockJob *job);
 
+/**
+ * block_job_pause_point:
+ * @job: The job that is ready to pause.
+ *
+ * Pause now if block_job_pause() has been called.  Block jobs that perform
+ * lots of I/O must call this between requests so that the job can be paused.
+ */
+void coroutine_fn block_job_pause_point(BlockJob *job);
+
 /**
  * block_job_pause:
  * @job: The job to be paused.