[Qemu-devel] [PATCH v3] blockjob: Fix hang in block_job_finish_sync

[Fam Zheng <famz@redhat.com>]

With a mirror job running on a virtio-blk dataplane disk, sending "q" to
HMP will cause a dead loop in block_job_finish_sync.

This is because the aio_poll() only processes the AIO context of bs
which has no more work to do, while the main loop BH that is scheduled
for setting the job->completed flag is never processed.

Fix this by adding a flag in BlockJob structure, to track which context
to poll for the block job to make progress. Its value is set to true
when block_job_coroutine_complete() is called, and is checked in
block_job_finish_sync to determine which context to poll.

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
---
 blockjob.c               | 5 ++++-
 include/block/blockjob.h | 9 +++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/blockjob.c b/blockjob.c
index 80adb9d..25e1581 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -304,7 +304,9 @@ static int block_job_finish_sync(BlockJob *job,
         return -EBUSY;
     }
     while (!job->completed) {
-        aio_poll(bdrv_get_aio_context(bs), true);
+        aio_poll(job->deferred_to_main_loop ? qemu_get_aio_context() :
+                                              bdrv_get_aio_context(bs),
+                 true);
     }
     ret = (job->cancelled && job->ret == 0) ? -ECANCELED : job->ret;
     block_job_unref(job);
@@ -497,6 +499,7 @@ void block_job_defer_to_main_loop(BlockJob *job,
     data->aio_context = bdrv_get_aio_context(job->bs);
     data->fn = fn;
     data->opaque = opaque;
+    job->deferred_to_main_loop = true;
 
     qemu_bh_schedule(data->bh);
 }
diff --git a/include/block/blockjob.h b/include/block/blockjob.h
index d84ccd8..550de26 100644
--- a/include/block/blockjob.h
+++ b/include/block/blockjob.h
@@ -130,6 +130,11 @@ struct BlockJob {
      */
     bool ready;
 
+    /**
+     * Set to true when the job has deferred work to the main loop.
+     */
+    bool deferred_to_main_loop;
+
     /** Status that is published by the query-block-jobs QMP API */
     BlockDeviceIoStatus iostatus;
 
@@ -402,6 +407,10 @@ typedef void BlockJobDeferToMainLoopFn(BlockJob *job, void *opaque);
  * AioContext acquired.  Block jobs must call bdrv_unref(), bdrv_close(), and
  * anything that uses bdrv_drain_all() in the main loop.
  *
+ * The job->deferred_to_main_loop flag will be set. Caller must clear it once
+ * the deferred work is done and the block job coroutine continues, unless it's
+ * completing immediately.
+ *
  * The @job AioContext is held while @fn executes.
  */
 void block_job_defer_to_main_loop(BlockJob *job,
-- 
2.4.3