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[PATCH v2 3/5] nbd: make nbd_export_close_all() synchronous

From: Vladimir Sementsov-Ogievskiy
Subject: [PATCH v2 3/5] nbd: make nbd_export_close_all() synchronous
Date: Wed, 1 Jul 2020 13:53:29 +0300

Consider nbd_export_close_all(). The call-stack looks like this:
 nbd_export_close_all() -> nbd_export_close -> call client_close() for
each client.

client_close() doesn't guarantee that client is closed: nbd_trip()
keeps reference to it. So, nbd_export_close_all() just reduce
reference counter on export and removes it from the list, but doesn't
guarantee that nbd_trip() finished neither export actually removed.

Let's wait for all exports actually removed.

Without this fix, the following crash is possible:

- export bitmap through internal Qemu NBD server
- connect a client
- shutdown Qemu

On shutdown nbd_export_close_all is called, but it actually don't wait
for nbd_trip() to finish and to release its references. So, export is
not release, and exported bitmap remains busy, and on try to remove the
bitmap (which is part of bdrv_close()) the assertion fails:

bdrv_release_dirty_bitmap_locked: Assertion `!bdrv_dirty_bitmap_busy(bitmap)' 

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

v2: rewritten, try to wait exports directly.

Note: I'm not sure in my understanding of AIO_WAIT_WHILE and related things
and really hope for review.

 nbd/server.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/nbd/server.c b/nbd/server.c
index 20754e9ebc..9d64b00f4b 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -102,6 +102,8 @@ struct NBDExport {
 static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);
+static QTAILQ_HEAD(, NBDExport) closed_exports =
+        QTAILQ_HEAD_INITIALIZER(closed_exports);
 /* NBDExportMetaContexts represents a list of contexts to be exported,
  * as selected by NBD_OPT_SET_META_CONTEXT. Also used for
@@ -1655,6 +1657,7 @@ void nbd_export_close(NBDExport *exp)
         exp->name = NULL;
         QTAILQ_REMOVE(&exports, exp, next);
+        QTAILQ_INSERT_TAIL(&closed_exports, exp, next);
     exp->description = NULL;
@@ -1717,7 +1720,9 @@ void nbd_export_put(NBDExport *exp)
+        QTAILQ_REMOVE(&closed_exports, exp, next);
+        aio_wait_kick();
@@ -1737,6 +1742,9 @@ void nbd_export_close_all(void)
+                           QTAILQ_EMPTY(&closed_exports)));
 static int coroutine_fn nbd_co_send_iov(NBDClient *client, struct iovec *iov,

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