[Qemu-block] [RFC PATCH 12/12] mirror: Protect source between bdrv_drain and bdrv_swap

[Fam Zheng]

Source and target are in sync when we leave the mirror_run loop, they
should remain so until bdrv_swap. Before block_job_defer_to_main_loop
was introduced, it has been easy to prove that. Now that tricky things
can happen after mirror_run returns and before mirror_exit runs, for
example, ioeventfd handlers being called, or a device model timer
callback submitting more I/O.

So, skip the block_job_defer_to_main_loop if we're already in the main
context. This is a necessary special casing until BlockBackend really
honors bdrv_lock.

If we're not in the main context, we rely on the notifying mechanism to
do the right thing.

Signed-off-by: Fam Zheng <address@hidden>
---
 block/mirror.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index 58f391a..24cf687 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -57,6 +57,8 @@ typedef struct MirrorBlockJob {
     int in_flight;
     int sectors_in_flight;
     int ret;
+    /* True if the source is locked by us */
+    bool need_unlock;
 } MirrorBlockJob;
 
 typedef struct MirrorOp {
@@ -358,6 +360,9 @@ static void mirror_exit(BlockJob *job, void *opaque)
     if (replace_aio_context) {
         aio_context_release(replace_aio_context);
     }
+    if (s->need_unlock) {
+        bdrv_unlock(s->common.bs);
+    }
     g_free(s->replaces);
     bdrv_unref(s->target);
     block_job_completed(&s->common, data->ret);
@@ -521,7 +526,8 @@ static void coroutine_fn mirror_run(void *opaque)
              * mirror_populate runs.
              */
             trace_mirror_before_drain(s, cnt);
-            bdrv_drain(bs);
+            bdrv_lock(bs);
+            s->need_unlock = true;
             cnt = bdrv_get_dirty_count(s->dirty_bitmap);
         }
 
@@ -543,6 +549,10 @@ static void coroutine_fn mirror_run(void *opaque)
             s->common.cancelled = false;
             break;
         }
+        if (s->need_unlock) {
+            bdrv_unlock(bs);
+            s->need_unlock = false;
+        }
         last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     }
 
@@ -565,7 +575,11 @@ immediate_exit:
 
     data = g_malloc(sizeof(*data));
     data->ret = ret;
-    block_job_defer_to_main_loop(&s->common, mirror_exit, data);
+    if (bs->aio_context == qemu_get_aio_context()) {
+        mirror_exit(&s->common, data);
+    } else {
+        block_job_defer_to_main_loop(&s->common, mirror_exit, data);
+    }
 }
 
 static void mirror_set_speed(BlockJob *job, int64_t speed, Error **errp)
-- 
2.4.2