block: let commit blockjob run in BDS AioContext

The commit block job must run in the BlockDriverState AioContext so that
it works with dataplane.

Acquire the AioContext in blockdev.c so starting the block job is safe.
One detail here is that the bdrv_drain_all() must be moved inside the
aio_context_acquire() region so requests cannot sneak in between the
drain and acquire.

The completion code in block/commit.c must perform backing chain
manipulation and bdrv_reopen() from the main loop.  Use
block_job_defer_to_main_loop() to achieve that.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1413889440-32577-11-git-send-email-stefanha@redhat.com

1 files changed, 20 insertions, 9 deletions

diff --git a/blockdev.c b/blockdev.c
index 13763225ee..57910b82c7 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2027,6 +2027,7 @@ void qmp_block_commit(const char *device,
 {
     BlockDriverState *bs;
     BlockDriverState *base_bs, *top_bs;
+    AioContext *aio_context;
     Error *local_err = NULL;
     /* This will be part of the QMP command, if/when the
      * BlockdevOnError change for blkmirror makes it in
@@ -2037,9 +2038,6 @@ void qmp_block_commit(const char *device,
         speed = 0;
     }
 
-    /* drain all i/o before commits */
-    bdrv_drain_all();
-
     /* Important Note:
      *  libvirt relies on the DeviceNotFound error class in order to probe for
      *  live commit feature versions; for this to work, we must make sure to
@@ -2051,8 +2049,14 @@ void qmp_block_commit(const char *device,
         return;
     }
 
+    aio_context = bdrv_get_aio_context(bs);
+    aio_context_acquire(aio_context);
+
+    /* drain all i/o before commits */
+    bdrv_drain_all();
+
     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_COMMIT, errp)) {
-        return;
+        goto out;
     }
 
     /* default top_bs is the active layer */
@@ -2066,9 +2070,11 @@ void qmp_block_commit(const char *device,
 
     if (top_bs == NULL) {
         error_setg(errp, "Top image file %s not found", top ? top : "NULL");
-        return;
+        goto out;
     }
 
+    assert(bdrv_get_aio_context(top_bs) == aio_context);
+
     if (has_base && base) {
         base_bs = bdrv_find_backing_image(top_bs, base);
     } else {
@@ -2077,20 +2083,22 @@ void qmp_block_commit(const char *device,
 
     if (base_bs == NULL) {
         error_set(errp, QERR_BASE_NOT_FOUND, base ? base : "NULL");
-        return;
+        goto out;
     }
 
+    assert(bdrv_get_aio_context(base_bs) == aio_context);
+
     /* Do not allow attempts to commit an image into itself */
     if (top_bs == base_bs) {
         error_setg(errp, "cannot commit an image into itself");
-        return;
+        goto out;
     }
 
     if (top_bs == bs) {
         if (has_backing_file) {
             error_setg(errp, "'backing-file' specified,"
                              " but 'top' is the active layer");
-            return;
+            goto out;
         }
         commit_active_start(bs, base_bs, speed, on_error, block_job_cb,
                             bs, &local_err);
@@ -2100,8 +2108,11 @@ void qmp_block_commit(const char *device,
     }
     if (local_err != NULL) {
         error_propagate(errp, local_err);
-        return;
+        goto out;
     }
+
+out:
+    aio_context_release(aio_context);
 }
 
 void qmp_drive_backup(const char *device, const char *target,