rbd-nbd: fix potential race when waiting unqiesce

There was a window between issuing quiesce_complete and checking
the quiesce variable unset by the unqiesce callback, when the lock
was not holding. If during that window the unquiesce following the
next quiesce callbacks were called we would miss unquiesce event.

Signed-off-by: Mykola Golub <[email protected]>

src/tools/rbd_nbd/rbd-nbd.cc

@@ -424,10 +424,9 @@ class NBDServer
     return true;
   }
 
-  void wait_unquiesce() {
+  void wait_unquiesce(std::unique_lock<ceph::mutex> &locker) {
     dout(20) << __func__ << dendl;
 
-    std::unique_lock locker{lock};
     cond.wait(locker, [this] { return !quiesce || terminated; });
 
     dout(20) << __func__ << ": got unquiesce request" << dendl;
@@ -466,10 +465,14 @@ class NBDServer
 
       wait_inflight_io();
 
-      // TODO: return quiesce hook exit code
-      image.quiesce_complete(quiesce_watch_handle, 0);
+      {
+        std::unique_lock locker{lock};
+
+        // TODO: return quiesce hook exit code
+        image.quiesce_complete(quiesce_watch_handle, 0);
 
-      wait_unquiesce();
+        wait_unquiesce(locker);
+      }
 
       run_quiesce_hook(cfg->quiesce_hook, cfg->devpath, "unquiesce");
     }