block: Introduce bdrv_schedule_unref()

bdrv_unref() is called by a lot of places that need to hold the graph
lock (it naturally happens in the context of operations that change the
graph). However, bdrv_unref() takes the graph writer lock internally, so
it can't actually be called while already holding a graph lock without
causing a deadlock.

bdrv_unref() also can't just become GRAPH_WRLOCK because it drains the
node before closing it, and draining requires that the graph is
unlocked.

The solution is to defer deleting the node until we don't hold the lock
any more and draining is possible again.

Note that keeping images open for longer than necessary can create
problems, too: You can't open an image again before it is really closed
(if image locking didn't prevent it, it would cause corruption).
Reopening an image immediately happens at least during bdrv_open() and
bdrv_co_create().

In order to solve this problem, make sure to run the deferred unref in
bdrv_graph_wrunlock(), i.e. the first possible place where we can drain
again. This is also why bdrv_schedule_unref() is marked GRAPH_WRLOCK.

The output of iotest 051 is updated because the additional polling
changes the order of HMP output, resulting in a new "(qemu)" prompt in
the test output that was previously on a separate line and filtered out.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230911094620.45040-6-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 19 insertions, 7 deletions

diff --git a/block/graph-lock.c b/block/graph-lock.c
index f357a2c0b1..58a799065f 100644
--- a/block/graph-lock.c
+++ b/block/graph-lock.c
@@ -163,17 +163,29 @@ void bdrv_graph_wrlock(BlockDriverState *bs)
 void bdrv_graph_wrunlock(void)
 {
     GLOBAL_STATE_CODE();
-    QEMU_LOCK_GUARD(&aio_context_list_lock);
     assert(qatomic_read(&has_writer));
 
+    WITH_QEMU_LOCK_GUARD(&aio_context_list_lock) {
+        /*
+         * No need for memory barriers, this works in pair with
+         * the slow path of rdlock() and both take the lock.
+         */
+        qatomic_store_release(&has_writer, 0);
+
+        /* Wake up all coroutines that are waiting to read the graph */
+        qemu_co_enter_all(&reader_queue, &aio_context_list_lock);
+    }
+
     /*
-     * No need for memory barriers, this works in pair with
-     * the slow path of rdlock() and both take the lock.
+     * Run any BHs that were scheduled during the wrlock section and that
+     * callers might expect to have finished (in particular, this is important
+     * for bdrv_schedule_unref()).
+     *
+     * Do this only after restarting coroutines so that nested event loops in
+     * BHs don't deadlock if their condition relies on the coroutine making
+     * progress.
      */
-    qatomic_store_release(&has_writer, 0);
-
-    /* Wake up all coroutine that are waiting to read the graph */
-    qemu_co_enter_all(&reader_queue, &aio_context_list_lock);
+    aio_bh_poll(qemu_get_aio_context());
 }
 
 void coroutine_fn bdrv_graph_co_rdlock(void)