nbd: keep send_mutex/free_sema handling outside nbd_co_do_establish_connection

Elevate s->in_flight early so that other incoming requests will wait
on the CoQueue in nbd_co_send_request; restart them after getting back
from nbd_reconnect_attempt.  This could be after the reconnect timer or
nbd_cancel_in_flight have cancelled the attempt, so there is no
need anymore to cancel the requests there.

nbd_co_send_request now handles both stopping and restarting pending
requests after a successful connection, and there is no need to
hold send_mutex in nbd_co_do_establish_connection.  The current setup
is confusing because nbd_co_do_establish_connection is called both with
send_mutex taken and without it.  Before the patch it uses free_sema which
(at least in theory...) is protected by send_mutex, after the patch it
does not anymore.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220414175756.671165-5-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: wrap long line]
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>
Signed-off-by: Eric Blake <eblake@redhat.com>

block/coroutines.h

@@ -59,7 +59,8 @@ int coroutine_fn bdrv_co_writev_vmstate(BlockDriverState *bs,
                                         QEMUIOVector *qiov, int64_t pos);
 
 int coroutine_fn
-nbd_co_do_establish_connection(BlockDriverState *bs, Error **errp);
+nbd_co_do_establish_connection(BlockDriverState *bs, bool blocking,
+                               Error **errp);
 
 
 int coroutine_fn
@@ -109,7 +110,7 @@ bdrv_common_block_status_above(BlockDriverState *bs,
                                BlockDriverState **file,
                                int *depth);
 int generated_co_wrapper
-nbd_do_establish_connection(BlockDriverState *bs, Error **errp);
+nbd_do_establish_connection(BlockDriverState *bs, bool blocking, Error **errp);
 
 int generated_co_wrapper
 blk_do_preadv(BlockBackend *blk, int64_t offset, int64_t bytes,

block/nbd.c

@@ -187,9 +187,6 @@ static void reconnect_delay_timer_cb(void *opaque)
     if (qatomic_load_acquire(&s->state) == NBD_CLIENT_CONNECTING_WAIT) {
         s->state = NBD_CLIENT_CONNECTING_NOWAIT;
         nbd_co_establish_connection_cancel(s->conn);
-        while (qemu_co_enter_next(&s->free_sema, NULL)) {
-            /* Resume all queued requests */
-        }
     }
 
     reconnect_delay_timer_del(s);
@@ -310,11 +307,10 @@ static int nbd_handle_updated_info(BlockDriverState *bs, Error **errp)
 }
 
 int coroutine_fn nbd_co_do_establish_connection(BlockDriverState *bs,
-                                                Error **errp)
+                                                bool blocking, Error **errp)
 {
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
     int ret;
-    bool blocking = nbd_client_connecting_wait(s);
     IO_CODE();
 
     assert(!s->ioc);
@@ -350,7 +346,6 @@ int coroutine_fn nbd_co_do_establish_connection(BlockDriverState *bs,
 
     /* successfully connected */
     s->state = NBD_CLIENT_CONNECTED;
-    qemu_co_queue_restart_all(&s->free_sema);
 
     return 0;
 }
@@ -358,25 +353,25 @@ int coroutine_fn nbd_co_do_establish_connection(BlockDriverState *bs,
 /* called under s->send_mutex */
 static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
 {
-    assert(nbd_client_connecting(s));
+    bool blocking = nbd_client_connecting_wait(s);
-    assert(s->in_flight == 0);
-
-    if (nbd_client_connecting_wait(s) && s->reconnect_delay &&
-        !s->reconnect_delay_timer)
-    {
-        /*
-         * It's first reconnect attempt after switching to
-         * NBD_CLIENT_CONNECTING_WAIT
-         */
-        reconnect_delay_timer_init(s,
-            qemu_clock_get_ns(QEMU_CLOCK_REALTIME) +
-            s->reconnect_delay * NANOSECONDS_PER_SECOND);
-    }
 
     /*
      * Now we are sure that nobody is accessing the channel, and no one will
      * try until we set the state to CONNECTED.
      */
+    assert(nbd_client_connecting(s));
+    assert(s->in_flight == 1);
+
+    if (blocking && !s->reconnect_delay_timer) {
+        /*
+         * It's the first reconnect attempt after switching to
+         * NBD_CLIENT_CONNECTING_WAIT
+         */
+        g_assert(s->reconnect_delay);
+        reconnect_delay_timer_init(s,
+            qemu_clock_get_ns(QEMU_CLOCK_REALTIME) +
+            s->reconnect_delay * NANOSECONDS_PER_SECOND);
+    }
 
     /* Finalize previous connection if any */
     if (s->ioc) {
@@ -387,7 +382,9 @@ static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
         s->ioc = NULL;
     }
 
-    nbd_co_do_establish_connection(s->bs, NULL);
+    qemu_co_mutex_unlock(&s->send_mutex);
+    nbd_co_do_establish_connection(s->bs, blocking, NULL);
+    qemu_co_mutex_lock(&s->send_mutex);
 
     /*
      * The reconnect attempt is done (maybe successfully, maybe not), so
@@ -472,21 +469,21 @@ static int coroutine_fn nbd_co_send_request(BlockDriverState *bs,
     qemu_co_mutex_lock(&s->send_mutex);
 
     while (s->in_flight == MAX_NBD_REQUESTS ||
-           (!nbd_client_connected(s) && s->in_flight > 0))
+           (!nbd_client_connected(s) && s->in_flight > 0)) {
-    {
         qemu_co_queue_wait(&s->free_sema, &s->send_mutex);
     }
 
-    if (nbd_client_connecting(s)) {
-        nbd_reconnect_attempt(s);
-    }
-
-    if (!nbd_client_connected(s)) {
-        rc = -EIO;
-        goto err;
-    }
-
     s->in_flight++;
+    if (!nbd_client_connected(s)) {
+        if (nbd_client_connecting(s)) {
+            nbd_reconnect_attempt(s);
+            qemu_co_queue_restart_all(&s->free_sema);
+        }
+        if (!nbd_client_connected(s)) {
+            rc = -EIO;
+            goto err;
+        }
+    }
 
     for (i = 0; i < MAX_NBD_REQUESTS; i++) {
         if (s->requests[i].coroutine == NULL) {
@@ -526,8 +523,8 @@ err:
         nbd_channel_error(s, rc);
         if (i != -1) {
             s->requests[i].coroutine = NULL;
-            s->in_flight--;
         }
+        s->in_flight--;
         qemu_co_queue_next(&s->free_sema);
     }
     qemu_co_mutex_unlock(&s->send_mutex);
@@ -1882,7 +1879,7 @@ static int nbd_open(BlockDriverState *bs, QDict *options, int flags,
     }
 
     s->state = NBD_CLIENT_CONNECTING_WAIT;
-    ret = nbd_do_establish_connection(bs, errp);
+    ret = nbd_do_establish_connection(bs, true, errp);
     if (ret < 0) {
         goto fail;
     }
@@ -2048,7 +2045,6 @@ static void nbd_cancel_in_flight(BlockDriverState *bs)
 
     if (s->state == NBD_CLIENT_CONNECTING_WAIT) {
         s->state = NBD_CLIENT_CONNECTING_NOWAIT;
-        qemu_co_queue_restart_all(&s->free_sema);
     }
 
     nbd_co_establish_connection_cancel(s->conn);