[PULL 11/13] nbd: take receive_mutex when reading requests[].receiving

[Eric Blake <eblake@redhat.com>]

From: Paolo Bonzini <pbonzini@redhat.com>

requests[].receiving is set by nbd_receive_replies() under the receive_mutex;
Read it under the same mutex as well.  Waking up receivers on errors happens
after each reply finishes processing, in nbd_co_receive_one_chunk().
If there is no currently-active reply, there are two cases:

* either there is no active request at all, in which case no
element of request[] can have .receiving = true

* or nbd_receive_replies() must be running and owns receive_mutex;
in that case it will get back to nbd_co_receive_one_chunk() because
the socket has been shutdown, and all waiting coroutines will wake up
in turn.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220414175756.671165-9-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/nbd.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/block/nbd.c b/block/nbd.c
index a5c690cef76b..3bfcf4d97c42 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -131,6 +131,7 @@ static void nbd_clear_bdrvstate(BlockDriverState *bs)
     s->x_dirty_bitmap = NULL;
 }

+/* Called with s->receive_mutex taken.  */
 static bool coroutine_fn nbd_recv_coroutine_wake_one(NBDClientRequest *req)
 {
     if (req->receiving) {
@@ -142,12 +143,13 @@ static bool coroutine_fn nbd_recv_coroutine_wake_one(NBDClientRequest *req)
     return false;
 }

-static void coroutine_fn nbd_recv_coroutines_wake(BDRVNBDState *s, bool all)
+static void coroutine_fn nbd_recv_coroutines_wake(BDRVNBDState *s)
 {
     int i;

+    QEMU_LOCK_GUARD(&s->receive_mutex);
     for (i = 0; i < MAX_NBD_REQUESTS; i++) {
-        if (nbd_recv_coroutine_wake_one(&s->requests[i]) && !all) {
+        if (nbd_recv_coroutine_wake_one(&s->requests[i])) {
             return;
         }
     }
@@ -168,8 +170,6 @@ static void coroutine_fn nbd_channel_error_locked(BDRVNBDState *s, int ret)
     } else {
         s->state = NBD_CLIENT_QUIT;
     }
-
-    nbd_recv_coroutines_wake(s, true);
 }

 static void coroutine_fn nbd_channel_error(BDRVNBDState *s, int ret)
@@ -432,11 +432,10 @@ static coroutine_fn int nbd_receive_replies(BDRVNBDState *s, uint64_t handle)

             qemu_coroutine_yield();
             /*
-             * We may be woken for 3 reasons:
+             * We may be woken for 2 reasons:
              * 1. From this function, executing in parallel coroutine, when our
              *    handle is received.
-             * 2. From nbd_channel_error(), when connection is lost.
-             * 3. From nbd_co_receive_one_chunk(), when previous request is
+             * 2. From nbd_co_receive_one_chunk(), when previous request is
              *    finished and s->reply.handle set to 0.
              * Anyway, it's OK to lock the mutex and go to the next iteration.
              */
@@ -928,7 +927,7 @@ static coroutine_fn int nbd_co_receive_one_chunk(
     }
     s->reply.handle = 0;

-    nbd_recv_coroutines_wake(s, false);
+    nbd_recv_coroutines_wake(s);

     return ret;
 }
-- 
2.35.1