mirror: Allow target that already has a BlockBackend

We had to forbid mirroring to a target BDS that already had a BB
attached because the node swapping at job completion would add a second
BB and we didn't support multiple BBs on a single BDS at the time. Now
we do, so we can lift the restriction.

As we allow additional BlockBackends for the target, we must expect
other users to be sending requests. There may no requests be in flight
during the graph modification, so we have to drain those users now.

The core part of this patch is a revert of commit 40365552.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

1 files changed, 6 insertions, 27 deletions

diff --git a/block/mirror.c b/block/mirror.c
index efca8fc177..23aa10e5bf 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -20,7 +20,6 @@
 #include "qapi/qmp/qerror.h"
 #include "qemu/ratelimit.h"
 #include "qemu/bitmap.h"
-#include "qemu/error-report.h"
 
 #define SLICE_TIME    100000000ULL /* ns */
 #define MAX_IN_FLIGHT 16
@@ -466,24 +465,20 @@ static void mirror_exit(BlockJob *job, void *opaque)
             to_replace = s->to_replace;
         }
 
-        /* This was checked in mirror_start_job(), but meanwhile one of the
-         * nodes could have been newly attached to a BlockBackend. */
-        if (bdrv_has_blk(to_replace) && bdrv_has_blk(s->target)) {
-            error_report("block job: Can't create node with two BlockBackends");
-            data->ret = -EINVAL;
-            goto out;
-        }
-
         if (bdrv_get_flags(s->target) != bdrv_get_flags(to_replace)) {
             bdrv_reopen(s->target, bdrv_get_flags(to_replace), NULL);
         }
+
+        /* The mirror job has no requests in flight any more, but we need to
+         * drain potential other users of the BDS before changing the graph. */
+        bdrv_drained_begin(s->target);
         bdrv_replace_in_backing_chain(to_replace, s->target);
+        bdrv_drained_end(s->target);
+
         /* We just changed the BDS the job BB refers to */
         blk_remove_bs(job->blk);
         blk_insert_bs(job->blk, src);
     }
-
-out:
     if (s->to_replace) {
         bdrv_op_unblock_all(s->to_replace, s->replace_blocker);
         error_free(s->replace_blocker);
@@ -807,7 +802,6 @@ static void mirror_start_job(BlockDriverState *bs, BlockDriverState *target,
                              bool is_none_mode, BlockDriverState *base)
 {
     MirrorBlockJob *s;
-    BlockDriverState *replaced_bs;
 
     if (granularity == 0) {
         granularity = bdrv_get_default_bitmap_granularity(target);
@@ -824,21 +818,6 @@ static void mirror_start_job(BlockDriverState *bs, BlockDriverState *target,
         buf_size = DEFAULT_MIRROR_BUF_SIZE;
     }
 
-    /* We can't support this case as long as the block layer can't handle
-     * multiple BlockBackends per BlockDriverState. */
-    if (replaces) {
-        replaced_bs = bdrv_lookup_bs(replaces, replaces, errp);
-        if (replaced_bs == NULL) {
-            return;
-        }
-    } else {
-        replaced_bs = bs;
-    }
-    if (bdrv_has_blk(replaced_bs) && bdrv_has_blk(target)) {
-        error_setg(errp, "Can't create node with two BlockBackends");
-        return;
-    }
-
     s = block_job_create(driver, bs, speed, cb, opaque, errp);
     if (!s) {
         return;