[Qemu-block] [RFC PATCH 4/5] block: Drop AioContext lock in bdrv_drain_poll_top_level()

[Kevin Wolf]

Simimlar to AIO_WAIT_WHILE(), bdrv_drain_poll_top_level() needs to
release the AioContext lock of the node to be drained before calling
aio_poll(). Otherwise, callbacks called by aio_poll() would possibly
take the lock a second time and run into a deadlock with a nested
AIO_WAIT_WHILE() call.

Signed-off-by: Kevin Wolf <address@hidden>
---
 block/io.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/block/io.c b/block/io.c
index 7100344c7b..832d2536bf 100644
--- a/block/io.c
+++ b/block/io.c
@@ -268,9 +268,32 @@ bool bdrv_drain_poll(BlockDriverState *bs, bool recursive,
 static bool bdrv_drain_poll_top_level(BlockDriverState *bs, bool recursive,
                                       BdrvChild *ignore_parent)
 {
+    AioContext *ctx = bdrv_get_aio_context(bs);
+
+    /*
+     * We cannot easily release the lock unconditionally here because many
+     * callers of drain function (like qemu initialisation, tools, etc.) don't
+     * even hold the main context lock.
+     *
+     * This means that we fix potential deadlocks for the case where we are in
+     * the main context and polling a BDS in a different AioContext, but
+     * draining a BDS in the main context from a different I/O thread would
+     * still have this problem. Fortunately, this isn't supposed to happen
+     * anyway.
+     */
+    if (ctx != qemu_get_aio_context()) {
+        aio_context_release(ctx);
+    } else {
+        assert(qemu_get_current_aio_context() == qemu_get_aio_context());
+    }
+
     /* Execute pending BHs first and check everything else only after the BHs
      * have executed. */
-    while (aio_poll(bs->aio_context, false));
+    while (aio_poll(ctx, false));
+
+    if (ctx != qemu_get_aio_context()) {
+        aio_context_acquire(ctx);
+    }
 
     return bdrv_drain_poll(bs, recursive, ignore_parent, false);
 }
-- 
2.13.6