block/export: stop using is_external in vhost-user-blk server

vhost-user activity must be suspended during bdrv_drained_begin/end().
This prevents new requests from interfering with whatever is happening
in the drained section.

Previously this was done using aio_set_fd_handler()'s is_external
argument. In a multi-queue block layer world the aio_disable_external()
API cannot be used since multiple AioContext may be processing I/O, not
just one.

Switch to BlockDevOps->drained_begin/end() callbacks.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230516190238.8401-8-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 26 insertions, 2 deletions

diff --git a/block/export/vhost-user-blk-server.c b/block/export/vhost-user-blk-server.c
index f51a36a..81b5976 100644
--- a/block/export/vhost-user-blk-server.c
+++ b/block/export/vhost-user-blk-server.c
@@ -212,15 +212,21 @@ static void blk_aio_attached(AioContext *ctx, void *opaque)
 {
     VuBlkExport *vexp = opaque;
 
+    /*
+     * The actual attach will happen in vu_blk_drained_end() and we just
+     * restore ctx here.
+     */
     vexp->export.ctx = ctx;
-    vhost_user_server_attach_aio_context(&vexp->vu_server, ctx);
 }
 
 static void blk_aio_detach(void *opaque)
 {
     VuBlkExport *vexp = opaque;
 
-    vhost_user_server_detach_aio_context(&vexp->vu_server);
+    /*
+     * The actual detach already happened in vu_blk_drained_begin() but from
+     * this point on we must not access ctx anymore.
+     */
     vexp->export.ctx = NULL;
 }
 
@@ -272,6 +278,22 @@ static void vu_blk_exp_resize(void *opaque)
     vu_config_change_msg(&vexp->vu_server.vu_dev);
 }
 
+/* Called with vexp->export.ctx acquired */
+static void vu_blk_drained_begin(void *opaque)
+{
+    VuBlkExport *vexp = opaque;
+
+    vhost_user_server_detach_aio_context(&vexp->vu_server);
+}
+
+/* Called with vexp->export.blk AioContext acquired */
+static void vu_blk_drained_end(void *opaque)
+{
+    VuBlkExport *vexp = opaque;
+
+    vhost_user_server_attach_aio_context(&vexp->vu_server, vexp->export.ctx);
+}
+
 /*
  * Ensures that bdrv_drained_begin() waits until in-flight requests complete.
  *
@@ -285,6 +307,8 @@ static bool vu_blk_drained_poll(void *opaque)
 }
 
 static const BlockDevOps vu_blk_dev_ops = {
+    .drained_begin = vu_blk_drained_begin,
+    .drained_end   = vu_blk_drained_end,
     .drained_poll  = vu_blk_drained_poll,
     .resize_cb = vu_blk_exp_resize,
 };