[PATCH 3/3] block: Start/end drain on correct AioContext

[Hanna Reitz]

bdrv_parent_drained_{begin,end}_single() are supposed to operate on the
parent, not on the child, so they should not attempt to get the context
to poll from the child but the parent instead.  BDRV_POLL_WHILE(c->bs)
does get the context from the child, so we should replace it with
AIO_WAIT_WHILE() on the parent's context instead.

This problem becomes apparent when bdrv_replace_child_noperm() invokes
bdrv_parent_drained_end_single() after removing a child from a subgraph
that is in an I/O thread.  By the time bdrv_parent_drained_end_single()
is called, child->bs is NULL, and so BDRV_POLL_WHILE(c->bs, ...) will
poll the main loop instead of the I/O thread; but anything that
bdrv_parent_drained_end_single_no_poll() may have scheduled is going to
want to run in the I/O thread, but because we poll the main loop, the
I/O thread is never unpaused, and nothing is run, resulting in a
deadlock.

Closes: https://gitlab.com/qemu-project/qemu/-/issues/1215
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
---
 block/io.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/block/io.c b/block/io.c
index 0a8cbefe86..7df1129b3b 100644
--- a/block/io.c
+++ b/block/io.c
@@ -71,9 +71,10 @@ static void bdrv_parent_drained_end_single_no_poll(BdrvChild 
*c,
 void bdrv_parent_drained_end_single(BdrvChild *c)
 {
     int drained_end_counter = 0;
+    AioContext *ctx = bdrv_child_get_parent_aio_context(c);
     IO_OR_GS_CODE();
     bdrv_parent_drained_end_single_no_poll(c, &drained_end_counter);
-    BDRV_POLL_WHILE(c->bs, qatomic_read(&drained_end_counter) > 0);
+    AIO_WAIT_WHILE(ctx, qatomic_read(&drained_end_counter) > 0);
 }
 
 static void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore,
@@ -116,13 +117,14 @@ static bool bdrv_parent_drained_poll(BlockDriverState 
*bs, BdrvChild *ignore,
 
 void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll)
 {
+    AioContext *ctx = bdrv_child_get_parent_aio_context(c);
     IO_OR_GS_CODE();
     c->parent_quiesce_counter++;
     if (c->klass->drained_begin) {
         c->klass->drained_begin(c);
     }
     if (poll) {
-        BDRV_POLL_WHILE(c->bs, bdrv_parent_drained_poll_single(c));
+        AIO_WAIT_WHILE(ctx, bdrv_parent_drained_poll_single(c));
     }
 }
 
-- 
2.36.1