backup: follow AioContext change gracefully

Move s->target to the new AioContext when there is an AioContext change.

The backup_run() coroutine does not use asynchronous I/O so there is no
need to wait for in-flight requests in a BlockJobDriver->pause()
callback.

Guest writes are intercepted by the backup job.  Treat them as guest
activity and do it even while the job is paused.  This is necessary
since the only alternative would be to fail a job that experienced guest
writes during pause once the job is resumed.  In practice the guest
writes don't interfere with AioContext switching since bdrv_drain() is
used by bdrv_set_aio_context().

Loops already contain pause points because of block_job_sleep_ns() calls
in the yield_and_check() helper function.  It is necessary to convert a
raw qemu_coroutine_yield() to block_job_yield() so the
MIRROR_SYNC_MODE_NONE case can pause.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1466096189-6477-9-git-send-email-stefanha@redhat.com

1 files changed, 14 insertions, 8 deletions

diff --git a/block/backup.c b/block/backup.c
index feeb9f8bf2..581269b29a 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -246,12 +246,20 @@ static void backup_abort(BlockJob *job)
     }
 }
 
+static void backup_attached_aio_context(BlockJob *job, AioContext *aio_context)
+{
+    BackupBlockJob *s = container_of(job, BackupBlockJob, common);
+
+    blk_set_aio_context(s->target, aio_context);
+}
+
 static const BlockJobDriver backup_job_driver = {
-    .instance_size  = sizeof(BackupBlockJob),
-    .job_type       = BLOCK_JOB_TYPE_BACKUP,
-    .set_speed      = backup_set_speed,
-    .commit         = backup_commit,
-    .abort          = backup_abort,
+    .instance_size          = sizeof(BackupBlockJob),
+    .job_type               = BLOCK_JOB_TYPE_BACKUP,
+    .set_speed              = backup_set_speed,
+    .commit                 = backup_commit,
+    .abort                  = backup_abort,
+    .attached_aio_context   = backup_attached_aio_context,
 };
 
 static BlockErrorAction backup_error_action(BackupBlockJob *job,
@@ -392,9 +400,7 @@ static void coroutine_fn backup_run(void *opaque)
         while (!block_job_is_cancelled(&job->common)) {
             /* Yield until the job is cancelled.  We just let our before_write
              * notify callback service CoW requests. */
-            job->common.busy = false;
-            qemu_coroutine_yield();
-            job->common.busy = true;
+            block_job_yield(&job->common);
         }
     } else if (job->sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
         ret = backup_run_incremental(job);