[PATCH 6/7] block/nbd: decouple reconnect from drain

[Roman Kagan]

The reconnection logic doesn't need to stop while in a drained section.
Moreover it has to be active during the drained section, as the requests
that were caught in-flight with the connection to the server broken can
only usefully get drained if the connection is restored.  Otherwise such
requests can only either stall resulting in a deadlock (before
8c517de24a), or be aborted defeating the purpose of the reconnection
machinery (after 8c517de24a).

Since the pieces of the reconnection logic are now properly migrated
from one aio_context to another, it appears safe to just stop messing
with the drained section in the reconnection code.

Fixes: 5ad81b4946 ("nbd: Restrict connection_co reentrance")
Fixes: 8c517de24a ("block/nbd: fix drain dead-lock because of nbd 
reconnect-delay")
Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
---
 block/nbd.c | 79 +++--------------------------------------------------
 1 file changed, 4 insertions(+), 75 deletions(-)

diff --git a/block/nbd.c b/block/nbd.c
index a3eb9b9079..a5a9e4aca5 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -117,8 +117,6 @@ typedef struct BDRVNBDState {
     Coroutine *connection_co;
     Coroutine *teardown_co;
     QemuCoSleepState *connection_co_sleep_ns_state;
-    bool drained;
-    bool wait_drained_end;
     int in_flight;
     NBDClientState state;
     int connect_status;
@@ -311,12 +309,6 @@ static void nbd_client_attach_aio_context_bh(void *opaque)
     qemu_mutex_unlock(&thr->mutex);
 
     if (s->connection_co) {
-        /*
-         * The node is still drained, so we know the coroutine has yielded in
-         * nbd_read_eof(), the only place where bs->in_flight can reach 0, or
-         * it is entered for the first time. Both places are safe for entering
-         * the coroutine.
-         */
         qemu_aio_coroutine_enter(bs->aio_context, s->connection_co);
     }
     bdrv_dec_in_flight(bs);
@@ -344,37 +336,6 @@ static void nbd_client_attach_aio_context(BlockDriverState 
*bs,
     aio_wait_bh_oneshot(new_context, nbd_client_attach_aio_context_bh, bs);
 }
 
-static void coroutine_fn nbd_client_co_drain_begin(BlockDriverState *bs)
-{
-    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
-
-    s->drained = true;
-    if (s->connection_co_sleep_ns_state) {
-        qemu_co_sleep_wake(s->connection_co_sleep_ns_state);
-    }
-
-    nbd_co_establish_connection_cancel(bs, false);
-
-    reconnect_delay_timer_del(s);
-
-    if (qatomic_load_acquire(&s->state) == NBD_CLIENT_CONNECTING_WAIT) {
-        s->state = NBD_CLIENT_CONNECTING_NOWAIT;
-        qemu_co_queue_restart_all(&s->free_sema);
-    }
-}
-
-static void coroutine_fn nbd_client_co_drain_end(BlockDriverState *bs)
-{
-    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
-
-    s->drained = false;
-    if (s->wait_drained_end) {
-        s->wait_drained_end = false;
-        aio_co_wake(s->connection_co);
-    }
-}
-
-
 static void nbd_teardown_connection(BlockDriverState *bs)
 {
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
@@ -686,16 +647,6 @@ static coroutine_fn void 
nbd_reconnect_attempt(BDRVNBDState *s)
 
     ret = nbd_client_handshake(s->bs, &local_err);
 
-    if (s->drained) {
-        s->wait_drained_end = true;
-        while (s->drained) {
-            /*
-             * We may be entered once from nbd_client_attach_aio_context_bh
-             * and then from nbd_client_co_drain_end. So here is a loop.
-             */
-            qemu_coroutine_yield();
-        }
-    }
     bdrv_inc_in_flight(s->bs);
 
 out:
@@ -724,26 +675,10 @@ static coroutine_fn void 
nbd_co_reconnect_loop(BDRVNBDState *s)
     nbd_reconnect_attempt(s);
 
     while (nbd_client_connecting(s)) {
-        if (s->drained) {
-            bdrv_dec_in_flight(s->bs);
-            s->wait_drained_end = true;
-            while (s->drained) {
-                /*
-                 * We may be entered once from nbd_client_attach_aio_context_bh
-                 * and then from nbd_client_co_drain_end. So here is a loop.
-                 */
-                qemu_coroutine_yield();
-            }
-            bdrv_inc_in_flight(s->bs);
-        } else {
-            qemu_co_sleep_ns_wakeable(QEMU_CLOCK_REALTIME, timeout,
-                                      &s->connection_co_sleep_ns_state);
-            if (s->drained) {
-                continue;
-            }
-            if (timeout < max_timeout) {
-                timeout *= 2;
-            }
+        qemu_co_sleep_ns_wakeable(QEMU_CLOCK_REALTIME, timeout,
+                                  &s->connection_co_sleep_ns_state);
+        if (timeout < max_timeout) {
+            timeout *= 2;
         }
 
         nbd_reconnect_attempt(s);
@@ -2548,8 +2483,6 @@ static BlockDriver bdrv_nbd = {
     .bdrv_getlength             = nbd_getlength,
     .bdrv_detach_aio_context    = nbd_client_detach_aio_context,
     .bdrv_attach_aio_context    = nbd_client_attach_aio_context,
-    .bdrv_co_drain_begin        = nbd_client_co_drain_begin,
-    .bdrv_co_drain_end          = nbd_client_co_drain_end,
     .bdrv_refresh_filename      = nbd_refresh_filename,
     .bdrv_co_block_status       = nbd_client_co_block_status,
     .bdrv_dirname               = nbd_dirname,
@@ -2577,8 +2510,6 @@ static BlockDriver bdrv_nbd_tcp = {
     .bdrv_getlength             = nbd_getlength,
     .bdrv_detach_aio_context    = nbd_client_detach_aio_context,
     .bdrv_attach_aio_context    = nbd_client_attach_aio_context,
-    .bdrv_co_drain_begin        = nbd_client_co_drain_begin,
-    .bdrv_co_drain_end          = nbd_client_co_drain_end,
     .bdrv_refresh_filename      = nbd_refresh_filename,
     .bdrv_co_block_status       = nbd_client_co_block_status,
     .bdrv_dirname               = nbd_dirname,
@@ -2606,8 +2537,6 @@ static BlockDriver bdrv_nbd_unix = {
     .bdrv_getlength             = nbd_getlength,
     .bdrv_detach_aio_context    = nbd_client_detach_aio_context,
     .bdrv_attach_aio_context    = nbd_client_attach_aio_context,
-    .bdrv_co_drain_begin        = nbd_client_co_drain_begin,
-    .bdrv_co_drain_end          = nbd_client_co_drain_end,
     .bdrv_refresh_filename      = nbd_refresh_filename,
     .bdrv_co_block_status       = nbd_client_co_block_status,
     .bdrv_dirname               = nbd_dirname,
-- 
2.30.2