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[Qemu-block] [PATCH 1/2] block: Drain source node in bdrv_replace_node()

From: Kevin Wolf
Subject: [Qemu-block] [PATCH 1/2] block: Drain source node in bdrv_replace_node()
Date: Tue, 21 May 2019 21:16:37 +0200

Instead of just asserting that no requests are in flight in
bdrv_replace_node(), which is a requirement that most callers ignore, we
can just drain the source node right there. This fixes at least starting
a commit job while I/O is active on the backing chain, but probably
other callers, too.

Having requests in flight on the target node isn't a problem because the
target just gets new parents, but the call path of running requests
isn't modified. So we can just drop this assertion without a replacement.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1711643
Signed-off-by: Kevin Wolf <address@hidden>
 block.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index cb11537029..75f370dbba 100644
--- a/block.c
+++ b/block.c
@@ -4021,13 +4021,13 @@ void bdrv_replace_node(BlockDriverState *from, 
BlockDriverState *to,
     uint64_t perm = 0, shared = BLK_PERM_ALL;
     int ret;
-    assert(!atomic_read(&from->in_flight));
-    assert(!atomic_read(&to->in_flight));
     /* Make sure that @from doesn't go away until we have successfully attached
      * all of its parents to @to. */
+    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
+    bdrv_drained_begin(from);
     /* Put all parents into @list and calculate their cumulative permissions */
     QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
         assert(c->bs == from);
@@ -4068,6 +4068,7 @@ void bdrv_replace_node(BlockDriverState *from, 
BlockDriverState *to,
+    bdrv_drained_end(from);

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