[PULL 34/34] block/nbd: safer transition to receiving request

[Eric Blake <eblake@redhat.com>]

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

req->receiving is a flag of request being in one concrete yield point
in nbd_co_do_receive_one_chunk().

Such kind of boolean flag is always better to unset before scheduling
the coroutine, to avoid double scheduling. So, let's be more careful.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210610100802.5888-33-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/nbd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/nbd.c b/block/nbd.c
index 5cfb749e089d..3cbee762de8d 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -150,6 +150,7 @@ static void nbd_recv_coroutines_wake_all(BDRVNBDState *s)
         NBDClientRequest *req = &s->requests[i];

         if (req->coroutine && req->receiving) {
+            req->receiving = false;
             aio_co_wake(req->coroutine);
         }
     }
@@ -548,6 +549,7 @@ static coroutine_fn void nbd_connection_entry(void *opaque)
          *   connection_co happens through a bottom half, which can only
          *   run after we yield.
          */
+        s->requests[i].receiving = false;
         aio_co_wake(s->requests[i].coroutine);
         qemu_coroutine_yield();
     }
@@ -934,7 +936,7 @@ static coroutine_fn int nbd_co_do_receive_one_chunk(
     /* Wait until we're woken up by nbd_connection_entry.  */
     s->requests[i].receiving = true;
     qemu_coroutine_yield();
-    s->requests[i].receiving = false;
+    assert(!s->requests[i].receiving);
     if (!nbd_client_connected(s)) {
         error_setg(errp, "Connection closed");
         return -EIO;
-- 
2.31.1