[PATCH v2 for-5.1? 0/5] Fix nbd reconnect dead-locks

[PATCH v2 for-5.1? 0/5] Fix nbd reconnect dead-locks

[Vladimir Sementsov-Ogievskiy]

Hi all!

v2: it's a bit updated "[PATCH for-5.1? 0/3] Fix nbd reconnect dead-locks"
plus completely rewritten "[PATCH for-5.1? 0/4] non-blocking connect"
(which is now the only one patch 05)

01: new
02: rebased on 01, fix (add outer "if")
03-04: add Eric's r-b:
05: new

If 05 is too big for 5.1, it's OK to take only 01-04 or less, as well as
postponing everything to 5.2, as it's all not a degradation of 5.1
(it's a degradation of 4.2, together with the whole reconnect feature).

Vladimir Sementsov-Ogievskiy (5):
  block/nbd: split nbd_establish_connection out of nbd_client_connect
  block/nbd: allow drain during reconnect attempt
  block/nbd: on shutdown terminate connection attempt
  block/nbd: nbd_co_reconnect_loop(): don't sleep if drained
  block/nbd: use non-blocking connect: fix vm hang on connect()

 block/nbd.c        | 360 +++++++++++++++++++++++++++++++++++++++++----
 block/trace-events |   4 +-
 2 files changed, 331 insertions(+), 33 deletions(-)

-- 
2.21.0

[PATCH v2 1/5] block/nbd: split nbd_establish_connection out of nbd_client_connect

[Vladimir Sementsov-Ogievskiy]

We are going to implement non-blocking version of
nbd_establish_connection, which for a while will be used only for
nbd_reconnect_attempt, not for nbd_open, so we need to call it
separately.

Refactor nbd_reconnect_attempt in a way which makes next commit
simpler.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/nbd.c        | 60 +++++++++++++++++++++++++++-------------------
 block/trace-events |  4 ++--
 2 files changed, 38 insertions(+), 26 deletions(-)

diff --git a/block/nbd.c b/block/nbd.c
index 65a4f56924..2ec6623c18 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -93,7 +93,10 @@ typedef struct BDRVNBDState {
     char *x_dirty_bitmap;
 } BDRVNBDState;
 
-static int nbd_client_connect(BlockDriverState *bs, Error **errp);
+static QIOChannelSocket *nbd_establish_connection(SocketAddress *saddr,
+                                                  Error **errp);
+static int nbd_client_handshake(BlockDriverState *bs, QIOChannelSocket *sioc,
+                                Error **errp);
 
 static void nbd_clear_bdrvstate(BDRVNBDState *s)
 {
@@ -241,7 +244,9 @@ static bool nbd_client_connecting_wait(BDRVNBDState *s)
 
 static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
 {
+    int ret;
     Error *local_err = NULL;
+    QIOChannelSocket *sioc;
 
     if (!nbd_client_connecting(s)) {
         return;
@@ -280,19 +285,25 @@ static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
         s->ioc = NULL;
     }
 
-    s->connect_status = nbd_client_connect(s->bs, &local_err);
+    sioc = nbd_establish_connection(s->saddr, &local_err);
+    if (!sioc) {
+        ret = -ECONNREFUSED;
+        goto out;
+    }
+
+    ret = nbd_client_handshake(s->bs, sioc, &local_err);
+
+out:
+    s->connect_status = ret;
     error_free(s->connect_err);
     s->connect_err = NULL;
     error_propagate(&s->connect_err, local_err);
 
-    if (s->connect_status < 0) {
-        /* failed attempt */
-        return;
+    if (ret >= 0) {
+        /* successfully connected */
+        s->state = NBD_CLIENT_CONNECTED;
+        qemu_co_queue_restart_all(&s->free_sema);
     }
-
-    /* successfully connected */
-    s->state = NBD_CLIENT_CONNECTED;
-    qemu_co_queue_restart_all(&s->free_sema);
 }
 
 static coroutine_fn void nbd_co_reconnect_loop(BDRVNBDState *s)
@@ -1425,24 +1436,15 @@ static QIOChannelSocket *nbd_establish_connection(SocketAddress *saddr,
     return sioc;
 }
 
-static int nbd_client_connect(BlockDriverState *bs, Error **errp)
+/* nbd_client_handshake takes ownership on sioc. On failure it is unref'ed. */
+static int nbd_client_handshake(BlockDriverState *bs, QIOChannelSocket *sioc,
+                                Error **errp)
 {
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
     AioContext *aio_context = bdrv_get_aio_context(bs);
     int ret;
 
-    /*
-     * establish TCP connection, return error if it fails
-     * TODO: Configurable retry-until-timeout behaviour.
-     */
-    QIOChannelSocket *sioc = nbd_establish_connection(s->saddr, errp);
-
-    if (!sioc) {
-        return -ECONNREFUSED;
-    }
-
-    /* NBD handshake */
-    trace_nbd_client_connect(s->export);
+    trace_nbd_client_handshake(s->export);
     qio_channel_set_blocking(QIO_CHANNEL(sioc), false, NULL);
     qio_channel_attach_aio_context(QIO_CHANNEL(sioc), aio_context);
 
@@ -1489,7 +1491,7 @@ static int nbd_client_connect(BlockDriverState *bs, Error **errp)
         object_ref(OBJECT(s->ioc));
     }
 
-    trace_nbd_client_connect_success(s->export);
+    trace_nbd_client_handshake_success(s->export);
 
     return 0;
 
@@ -1894,6 +1896,7 @@ static int nbd_open(BlockDriverState *bs, QDict *options, int flags,
 {
     int ret;
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    QIOChannelSocket *sioc;
 
     ret = nbd_process_options(bs, options, errp);
     if (ret < 0) {
@@ -1904,7 +1907,16 @@ static int nbd_open(BlockDriverState *bs, QDict *options, int flags,
     qemu_co_mutex_init(&s->send_mutex);
     qemu_co_queue_init(&s->free_sema);
 
-    ret = nbd_client_connect(bs, errp);
+    /*
+     * establish TCP connection, return error if it fails
+     * TODO: Configurable retry-until-timeout behaviour.
+     */
+    sioc = nbd_establish_connection(s->saddr, errp);
+    if (!sioc) {
+        return -ECONNREFUSED;
+    }
+
+    ret = nbd_client_handshake(bs, sioc, errp);
     if (ret < 0) {
         nbd_clear_bdrvstate(s);
         return ret;
diff --git a/block/trace-events b/block/trace-events
index d3533ca896..9158335061 100644
--- a/block/trace-events
+++ b/block/trace-events
@@ -168,8 +168,8 @@ nbd_parse_blockstatus_compliance(const char *err) "ignoring extra data from non-
 nbd_structured_read_compliance(const char *type) "server sent non-compliant unaligned read %s chunk"
 nbd_read_reply_entry_fail(int ret, const char *err) "ret = %d, err: %s"
 nbd_co_request_fail(uint64_t from, uint32_t len, uint64_t handle, uint16_t flags, uint16_t type, const char *name, int ret, const char *err) "Request failed { .from = %" PRIu64", .len = %" PRIu32 ", .handle = %" PRIu64 ", .flags = 0x%" PRIx16 ", .type = %" PRIu16 " (%s) } ret = %d, err: %s"
-nbd_client_connect(const char *export_name) "export '%s'"
-nbd_client_connect_success(const char *export_name) "export '%s'"
+nbd_client_handshake(const char *export_name) "export '%s'"
+nbd_client_handshake_success(const char *export_name) "export '%s'"
 
 # ssh.c
 ssh_restart_coroutine(void *co) "co=%p"
-- 
2.21.0

Re: [PATCH v2 1/5] block/nbd: split nbd_establish_connection out of nbd_client_connect

[Eric Blake]

On 7/27/20 1:47 PM, Vladimir Sementsov-Ogievskiy wrote:
> We are going to implement non-blocking version of
> nbd_establish_connection, which for a while will be used only for
> nbd_reconnect_attempt, not for nbd_open, so we need to call it
> separately.
> 
> Refactor nbd_reconnect_attempt in a way which makes next commit
> simpler.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   block/nbd.c        | 60 +++++++++++++++++++++++++++-------------------
>   block/trace-events |  4 ++--
>   2 files changed, 38 insertions(+), 26 deletions(-)
> 

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

[PATCH v2 2/5] block/nbd: allow drain during reconnect attempt

[Vladimir Sementsov-Ogievskiy]

It should be to reenter qio_channel_yield() on io/channel read/write
path, so it's safe to reduce in_flight and allow attaching new aio
context. And no problem to allow drain itself: connection attempt is
not a guest request. Moreover, if remote server is down, we can hang
in negotiation, blocking drain section and provoking a dead lock.

How to reproduce the dead lock:

1. Create nbd-fault-injector.conf with the following contents:

[inject-error "mega1"]
event=data
io=readwrite
when=before

2. In one terminal run nbd-fault-injector in a loop, like this:

n=1; while true; do
    echo $n; ((n++));
    ./nbd-fault-injector.py 127.0.0.1:10000 nbd-fault-injector.conf;
done

3. In another terminal run qemu-io in a loop, like this:

n=1; while true; do
    echo $n; ((n++));
    ./qemu-io -c 'read 0 512' nbd://127.0.0.1:10000;
done

After some time, qemu-io will hang trying to drain, for example, like
this:

 #3 aio_poll (ctx=0x55f006bdd890, blocking=true) at
    util/aio-posix.c:600
 #4 bdrv_do_drained_begin (bs=0x55f006bea710, recursive=false,
    parent=0x0, ignore_bds_parents=false, poll=true) at block/io.c:427
 #5 bdrv_drained_begin (bs=0x55f006bea710) at block/io.c:433
 #6 blk_drain (blk=0x55f006befc80) at block/block-backend.c:1710
 #7 blk_unref (blk=0x55f006befc80) at block/block-backend.c:498
 #8 bdrv_open_inherit (filename=0x7fffba1563bc
    "nbd+tcp://127.0.0.1:10000", reference=0x0, options=0x55f006be86d0,
    flags=24578, parent=0x0, child_class=0x0, child_role=0,
    errp=0x7fffba154620) at block.c:3491
 #9 bdrv_open (filename=0x7fffba1563bc "nbd+tcp://127.0.0.1:10000",
    reference=0x0, options=0x0, flags=16386, errp=0x7fffba154620) at
    block.c:3513
 #10 blk_new_open (filename=0x7fffba1563bc "nbd+tcp://127.0.0.1:10000",
    reference=0x0, options=0x0, flags=16386, errp=0x7fffba154620) at
    block/block-backend.c:421

And connection_co stack like this:

 #0 qemu_coroutine_switch (from_=0x55f006bf2650, to_=0x7fe96e07d918,
    action=COROUTINE_YIELD) at util/coroutine-ucontext.c:302
 #1 qemu_coroutine_yield () at util/qemu-coroutine.c:193
 #2 qio_channel_yield (ioc=0x55f006bb3c20, condition=G_IO_IN) at
    io/channel.c:472
 #3 qio_channel_readv_all_eof (ioc=0x55f006bb3c20, iov=0x7fe96d729bf0,
    niov=1, errp=0x7fe96d729eb0) at io/channel.c:110
 #4 qio_channel_readv_all (ioc=0x55f006bb3c20, iov=0x7fe96d729bf0,
    niov=1, errp=0x7fe96d729eb0) at io/channel.c:143
 #5 qio_channel_read_all (ioc=0x55f006bb3c20, buf=0x7fe96d729d28
    "\300.\366\004\360U", buflen=8, errp=0x7fe96d729eb0) at
    io/channel.c:247
 #6 nbd_read (ioc=0x55f006bb3c20, buffer=0x7fe96d729d28, size=8,
    desc=0x55f004f69644 "initial magic", errp=0x7fe96d729eb0) at
    /work/src/qemu/master/include/block/nbd.h:365
 #7 nbd_read64 (ioc=0x55f006bb3c20, val=0x7fe96d729d28,
    desc=0x55f004f69644 "initial magic", errp=0x7fe96d729eb0) at
    /work/src/qemu/master/include/block/nbd.h:391
 #8 nbd_start_negotiate (aio_context=0x55f006bdd890,
    ioc=0x55f006bb3c20, tlscreds=0x0, hostname=0x0,
    outioc=0x55f006bf19f8, structured_reply=true,
    zeroes=0x7fe96d729dca, errp=0x7fe96d729eb0) at nbd/client.c:904
 #9 nbd_receive_negotiate (aio_context=0x55f006bdd890,
    ioc=0x55f006bb3c20, tlscreds=0x0, hostname=0x0,
    outioc=0x55f006bf19f8, info=0x55f006bf1a00, errp=0x7fe96d729eb0) at
    nbd/client.c:1032
 #10 nbd_client_connect (bs=0x55f006bea710, errp=0x7fe96d729eb0) at
    block/nbd.c:1460
 #11 nbd_reconnect_attempt (s=0x55f006bf19f0) at block/nbd.c:287
 #12 nbd_co_reconnect_loop (s=0x55f006bf19f0) at block/nbd.c:309
 #13 nbd_connection_entry (opaque=0x55f006bf19f0) at block/nbd.c:360
 #14 coroutine_trampoline (i0=113190480, i1=22000) at
    util/coroutine-ucontext.c:173

Note, that the hang may be
triggered by another bug, so the whole case is fixed only together with
commit "block/nbd: on shutdown terminate connection attempt".

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/nbd.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/block/nbd.c b/block/nbd.c
index 2ec6623c18..6d19f3c660 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -291,8 +291,22 @@ static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
         goto out;
     }
 
+    bdrv_dec_in_flight(s->bs);
+
     ret = nbd_client_handshake(s->bs, sioc, &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:
     s->connect_status = ret;
     error_free(s->connect_err);
-- 
2.21.0

Re: [PATCH v2 2/5] block/nbd: allow drain during reconnect attempt

[Eric Blake]

On 7/27/20 1:47 PM, Vladimir Sementsov-Ogievskiy wrote:
> It should be to reenter qio_channel_yield() on io/channel read/write

be safe

> path, so it's safe to reduce in_flight and allow attaching new aio
> context. And no problem to allow drain itself: connection attempt is
> not a guest request. Moreover, if remote server is down, we can hang
> in negotiation, blocking drain section and provoking a dead lock.
> 
> How to reproduce the dead lock:
> 
> 1. Create nbd-fault-injector.conf with the following contents:
> 
> [inject-error "mega1"]
> event=data
> io=readwrite
> when=before
> 
> 2. In one terminal run nbd-fault-injector in a loop, like this:
> 
> n=1; while true; do
>      echo $n; ((n++));
>      ./nbd-fault-injector.py 127.0.0.1:10000 nbd-fault-injector.conf;
> done
> 
> 3. In another terminal run qemu-io in a loop, like this:
> 
> n=1; while true; do
>      echo $n; ((n++));
>      ./qemu-io -c 'read 0 512' nbd://127.0.0.1:10000;
> done
> 

> 
> Note, that the hang may be
> triggered by another bug, so the whole case is fixed only together with
> commit "block/nbd: on shutdown terminate connection attempt".
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   block/nbd.c | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
> 
> diff --git a/block/nbd.c b/block/nbd.c
> index 2ec6623c18..6d19f3c660 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -291,8 +291,22 @@ static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
>           goto out;
>       }
>   
> +    bdrv_dec_in_flight(s->bs);
> +
>       ret = nbd_client_handshake(s->bs, sioc, &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:
>       s->connect_status = ret;
>       error_free(s->connect_err);
> 

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

[PATCH v2 3/5] block/nbd: on shutdown terminate connection attempt

[Vladimir Sementsov-Ogievskiy]

On shutdown nbd driver may be in a connecting state. We should shutdown
it as well, otherwise we may hang in
nbd_teardown_connection, waiting for conneciton_co to finish in
BDRV_POLL_WHILE(bs, s->connection_co) loop if remote server is down.

How to reproduce the dead lock:

1. Create nbd-fault-injector.conf with the following contents:

[inject-error "mega1"]
event=data
io=readwrite
when=before

2. In one terminal run nbd-fault-injector in a loop, like this:

n=1; while true; do
    echo $n; ((n++));
    ./nbd-fault-injector.py 127.0.0.1:10000 nbd-fault-injector.conf;
done

3. In another terminal run qemu-io in a loop, like this:

n=1; while true; do
    echo $n; ((n++));
    ./qemu-io -c 'read 0 512' nbd://127.0.0.1:10000;
done

After some time, qemu-io will hang. Note, that this hang may be
triggered by another bug, so the whole case is fixed only together with
commit "block/nbd: allow drain during reconnect attempt".

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/nbd.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/block/nbd.c b/block/nbd.c
index 6d19f3c660..dfe1408b2d 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -209,11 +209,15 @@ static void nbd_teardown_connection(BlockDriverState *bs)
 {
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
 
-    if (s->state == NBD_CLIENT_CONNECTED) {
+    if (s->ioc) {
         /* finish any pending coroutines */
-        assert(s->ioc);
         qio_channel_shutdown(s->ioc, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
+    } else if (s->sioc) {
+        /* abort negotiation */
+        qio_channel_shutdown(QIO_CHANNEL(s->sioc), QIO_CHANNEL_SHUTDOWN_BOTH,
+                             NULL);
     }
+
     s->state = NBD_CLIENT_QUIT;
     if (s->connection_co) {
         if (s->connection_co_sleep_ns_state) {
@@ -1459,6 +1463,9 @@ static int nbd_client_handshake(BlockDriverState *bs, QIOChannelSocket *sioc,
     int ret;
 
     trace_nbd_client_handshake(s->export);
+
+    s->sioc = sioc;
+
     qio_channel_set_blocking(QIO_CHANNEL(sioc), false, NULL);
     qio_channel_attach_aio_context(QIO_CHANNEL(sioc), aio_context);
 
@@ -1473,6 +1480,7 @@ static int nbd_client_handshake(BlockDriverState *bs, QIOChannelSocket *sioc,
     g_free(s->info.name);
     if (ret < 0) {
         object_unref(OBJECT(sioc));
+        s->sioc = NULL;
         return ret;
     }
     if (s->x_dirty_bitmap && !s->info.base_allocation) {
@@ -1498,8 +1506,6 @@ static int nbd_client_handshake(BlockDriverState *bs, QIOChannelSocket *sioc,
         }
     }
 
-    s->sioc = sioc;
-
     if (!s->ioc) {
         s->ioc = QIO_CHANNEL(sioc);
         object_ref(OBJECT(s->ioc));
@@ -1520,6 +1526,7 @@ static int nbd_client_handshake(BlockDriverState *bs, QIOChannelSocket *sioc,
         nbd_send_request(s->ioc ?: QIO_CHANNEL(sioc), &request);
 
         object_unref(OBJECT(sioc));
+        s->sioc = NULL;
 
         return ret;
     }
-- 
2.21.0

[PATCH v2 4/5] block/nbd: nbd_co_reconnect_loop(): don't sleep if drained

[Vladimir Sementsov-Ogievskiy]

We try to go to wakeable sleep, so that, if drain begins it will break
the sleep. But what if nbd_client_co_drain_begin() already called and
s->drained is already true? We'll go to sleep, and drain will have to
wait for the whole timeout. Let's improve it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/nbd.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/block/nbd.c b/block/nbd.c
index dfe1408b2d..8c5df68856 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -341,8 +341,6 @@ static coroutine_fn void nbd_co_reconnect_loop(BDRVNBDState *s)
             qemu_co_queue_restart_all(&s->free_sema);
         }
 
-        qemu_co_sleep_ns_wakeable(QEMU_CLOCK_REALTIME, timeout,
-                                  &s->connection_co_sleep_ns_state);
         if (s->drained) {
             bdrv_dec_in_flight(s->bs);
             s->wait_drained_end = true;
@@ -354,9 +352,12 @@ static coroutine_fn void nbd_co_reconnect_loop(BDRVNBDState *s)
                 qemu_coroutine_yield();
             }
             bdrv_inc_in_flight(s->bs);
-        }
-        if (timeout < max_timeout) {
-            timeout *= 2;
+        } else {
+            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);
-- 
2.21.0

[PATCH v2 5/5] block/nbd: use non-blocking connect: fix vm hang on connect()

[Vladimir Sementsov-Ogievskiy]

This make nbd connection_co to yield during reconnects, so that
reconnect doesn't hang up the main thread. This is very important in
case of unavailable nbd server host: connect() call may take a long
time, blocking the main thread (and due to reconnect, it will hang
again and again with small gaps of working time during pauses between
connection attempts).

Realization notes:

 - We don't want to implement non-blocking connect() over non-blocking
 socket, because getaddrinfo() doesn't have portable non-blocking
 realization anyway, so let's just use a thread for both getaddrinfo()
 and connect().

 - We can't use qio_channel_socket_connect_async (which behave
 similarly and start a thread to execute connect() call), as it's rely
 on someone iterating main loop (g_main_loop_run() or something like
 this), which is not always the case.

 - We can't use thread_pool_submit_co API, as thread pool waits for all
 threads to finish (but we don't want to wait for blocking reconnect
 attempt on shutdown.

 So, we just create the thread by hand. Some additional difficulties
 are:

 - We want our connect don't block drained sections and aio context
 switches. To achieve this, we make it possible to "cancel" synchronous
 wait for the connect (which is an coroutine yield actually), still,
 the thread continues in background, and it successful result may be
 reused on next reconnect attempt.

 - We don't want to wait for reconnect on shutdown, so there is
 CONNECT_THREAD_RUNNING_DETACHED thread state, which means that block
 layer not more interested in a result, and thread should close new
 connected socket on finish and free the state.

How to reproduce the bug, fixed with this commit:

1. Create an image on node1:
   qemu-img create -f qcow2 xx 100M

2. Start NBD server on node1:
   qemu-nbd xx

3. Start vm with second nbd disk on node2, like this:

  ./x86_64-softmmu/qemu-system-x86_64 -nodefaults -drive \
    file=/work/images/cent7.qcow2 -drive file=nbd+tcp://192.168.100.2 \
    -vnc :0 -qmp stdio -m 2G -enable-kvm -vga std

4. Access the vm through vnc (or some other way?), and check that NBD
   drive works:

   dd if=/dev/sdb of=/dev/null bs=1M count=10

   - the command should succeed.

5. Now, let's trigger nbd-reconnect loop in Qemu process. For this:

5.1 Kill NBD server on node1

5.2 run "dd if=/dev/sdb of=/dev/null bs=1M count=10" in the guest
    again. The command should fail and a lot of error messages about
    failing disk may appear as well.

    Now NBD client driver in Qemu tries to reconnect.
    Still, VM works well.

6. Make node1 unavailable on NBD port, so connect() from node2 will
   last for a long time:

   On node1 (Note, that 10809 is just a default NBD port):

   sudo iptables -A INPUT -p tcp --dport 10809 -j DROP

   After some time the guest hangs, and you may check in gdb that Qemu
   hangs in connect() call, issued from the main thread. This is the
   BUG.

7. Don't forget to drop iptables rule from your node1:

   sudo iptables -D INPUT -p tcp --dport 10809 -j DROP

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/nbd.c | 266 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 265 insertions(+), 1 deletion(-)

diff --git a/block/nbd.c b/block/nbd.c
index 8c5df68856..75352adf89 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -38,6 +38,7 @@
 
 #include "qapi/qapi-visit-sockets.h"
 #include "qapi/qmp/qstring.h"
+#include "qapi/clone-visitor.h"
 
 #include "block/qdict.h"
 #include "block/nbd.h"
@@ -62,6 +63,47 @@ typedef enum NBDClientState {
     NBD_CLIENT_QUIT
 } NBDClientState;
 
+typedef enum NBDConnectThreadState {
+/* No thread, no pending results */
+    CONNECT_THREAD_NONE,
+
+/* Thread is running, no results for now */
+    CONNECT_THREAD_RUNNING,
+
+/*
+ * Thread is running, but requestor exited. Thread should close the new socket
+ * and free the connect state on exit.
+ */
+    CONNECT_THREAD_RUNNING_DETACHED,
+
+/* Thread finished, results are stored in a state */
+    CONNECT_THREAD_FAIL,
+    CONNECT_THREAD_SUCCESS
+} NBDConnectThreadState;
+
+typedef struct NBDConnectThread {
+    /* Initialization constants */
+    SocketAddress *saddr; /* address to connect to */
+    /*
+     * Bottom half to schedule on completion. Scheduled only if bh_ctx is not
+     * NULL
+     */
+    QEMUBHFunc *bh_func;
+    void *bh_opaque;
+
+    /*
+     * Result of last attempt. Valid in FAIL and SUCCESS states.
+     * If you want to steal error, don't forget to set pointer to NULL.
+     */
+    QIOChannelSocket *sioc;
+    Error *err;
+
+    /* state and bh_ctx are protected by mutex */
+    QemuMutex mutex;
+    NBDConnectThreadState state; /* current state of the thread */
+    AioContext *bh_ctx; /* where to schedule bh (NULL means don't schedule) */
+} NBDConnectThread;
+
 typedef struct BDRVNBDState {
     QIOChannelSocket *sioc; /* The master data channel */
     QIOChannel *ioc; /* The current I/O channel which may differ (eg TLS) */
@@ -91,10 +133,17 @@ typedef struct BDRVNBDState {
     QCryptoTLSCreds *tlscreds;
     const char *hostname;
     char *x_dirty_bitmap;
+
+    bool wait_connect;
+    NBDConnectThread *connect_thread;
 } BDRVNBDState;
 
 static QIOChannelSocket *nbd_establish_connection(SocketAddress *saddr,
                                                   Error **errp);
+static QIOChannelSocket *nbd_co_establish_connection(BlockDriverState *bs,
+                                                     Error **errp);
+static void nbd_co_establish_connection_cancel(BlockDriverState *bs,
+                                               bool detach);
 static int nbd_client_handshake(BlockDriverState *bs, QIOChannelSocket *sioc,
                                 Error **errp);
 
@@ -191,6 +240,8 @@ static void coroutine_fn nbd_client_co_drain_begin(BlockDriverState *bs)
     if (s->connection_co_sleep_ns_state) {
         qemu_co_sleep_wake(s->connection_co_sleep_ns_state);
     }
+
+    nbd_co_establish_connection_cancel(bs, false);
 }
 
 static void coroutine_fn nbd_client_co_drain_end(BlockDriverState *bs)
@@ -223,6 +274,7 @@ static void nbd_teardown_connection(BlockDriverState *bs)
         if (s->connection_co_sleep_ns_state) {
             qemu_co_sleep_wake(s->connection_co_sleep_ns_state);
         }
+        nbd_co_establish_connection_cancel(bs, true);
     }
     if (qemu_in_coroutine()) {
         s->teardown_co = qemu_coroutine_self();
@@ -246,6 +298,216 @@ static bool nbd_client_connecting_wait(BDRVNBDState *s)
     return s->state == NBD_CLIENT_CONNECTING_WAIT;
 }
 
+static void connect_bh(void *opaque)
+{
+    BDRVNBDState *state = opaque;
+
+    assert(state->wait_connect);
+    state->wait_connect = false;
+    aio_co_wake(state->connection_co);
+}
+
+static void nbd_init_connect_thread(BDRVNBDState *s)
+{
+    s->connect_thread = g_new(NBDConnectThread, 1);
+
+    *s->connect_thread = (NBDConnectThread) {
+        .saddr = QAPI_CLONE(SocketAddress, s->saddr),
+        .state = CONNECT_THREAD_NONE,
+        .bh_func = connect_bh,
+        .bh_opaque = s
+    };
+
+    qemu_mutex_init(&s->connect_thread->mutex);
+}
+
+static void nbd_free_connect_thread(NBDConnectThread *thr)
+{
+    if (thr->sioc) {
+        qio_channel_close(QIO_CHANNEL(thr->sioc), NULL);
+    }
+    error_free(thr->err);
+    qapi_free_SocketAddress(thr->saddr);
+    g_free(thr);
+}
+
+static void *connect_thread_func(void *opaque)
+{
+    NBDConnectThread *thr = opaque;
+    int ret;
+    bool do_free = false;
+
+    thr->sioc = qio_channel_socket_new();
+
+    error_free(thr->err);
+    thr->err = NULL;
+    ret = qio_channel_socket_connect_sync(thr->sioc, thr->saddr, &thr->err);
+    if (ret < 0) {
+        object_unref(OBJECT(thr->sioc));
+        thr->sioc = NULL;
+    }
+
+    qemu_mutex_lock(&thr->mutex);
+
+    switch (thr->state) {
+    case CONNECT_THREAD_RUNNING:
+        thr->state = ret < 0 ? CONNECT_THREAD_FAIL : CONNECT_THREAD_SUCCESS;
+        if (thr->bh_ctx) {
+            aio_bh_schedule_oneshot(thr->bh_ctx, thr->bh_func, thr->bh_opaque);
+
+            /* play safe, don't reuse bh_ctx on further connection attempts */
+            thr->bh_ctx = NULL;
+        }
+        break;
+    case CONNECT_THREAD_RUNNING_DETACHED:
+        do_free = true;
+        break;
+    default:
+        abort();
+    }
+
+    qemu_mutex_unlock(&thr->mutex);
+
+    if (do_free) {
+        nbd_free_connect_thread(thr);
+    }
+
+    return NULL;
+}
+
+static QIOChannelSocket *coroutine_fn
+nbd_co_establish_connection(BlockDriverState *bs, Error **errp)
+{
+    QemuThread thread;
+    BDRVNBDState *s = bs->opaque;
+    QIOChannelSocket *res;
+    NBDConnectThread *thr = s->connect_thread;
+
+    qemu_mutex_lock(&thr->mutex);
+
+    switch (thr->state) {
+    case CONNECT_THREAD_FAIL:
+    case CONNECT_THREAD_NONE:
+        error_free(thr->err);
+        thr->err = NULL;
+        thr->state = CONNECT_THREAD_RUNNING;
+        qemu_thread_create(&thread, "nbd-connect",
+                           connect_thread_func, thr, QEMU_THREAD_DETACHED);
+        break;
+    case CONNECT_THREAD_SUCCESS:
+        /* Previous attempt finally succeeded in background */
+        thr->state = CONNECT_THREAD_NONE;
+        res = thr->sioc;
+        thr->sioc = NULL;
+        qemu_mutex_unlock(&thr->mutex);
+        return res;
+    case CONNECT_THREAD_RUNNING:
+        /* Already running, will wait */
+        break;
+    default:
+        abort();
+    }
+
+    thr->bh_ctx = qemu_get_current_aio_context();
+
+    qemu_mutex_unlock(&thr->mutex);
+
+
+    /*
+     * We are going to wait for connect-thread finish, but
+     * nbd_client_co_drain_begin() can interrupt.
+     *
+     * Note that wait_connect variable is not visible for connect-thread. It
+     * doesn't need mutex protection, it used only inside home aio context of
+     * bs.
+     */
+    s->wait_connect = true;
+    qemu_coroutine_yield();
+
+    qemu_mutex_lock(&thr->mutex);
+
+    switch (thr->state) {
+    case CONNECT_THREAD_SUCCESS:
+    case CONNECT_THREAD_FAIL:
+        thr->state = CONNECT_THREAD_NONE;
+        error_propagate(errp, thr->err);
+        thr->err = NULL;
+        res = thr->sioc;
+        thr->sioc = NULL;
+        break;
+    case CONNECT_THREAD_RUNNING:
+    case CONNECT_THREAD_RUNNING_DETACHED:
+        /*
+         * Obviously, drained section wants to start. Report the attempt as
+         * failed. Still connect thread is executing in background, and its
+         * result may be used for next connection attempt.
+         */
+        res = NULL;
+        error_setg(errp, "Connection attempt cancelled by other operation");
+        break;
+
+    case CONNECT_THREAD_NONE:
+        /*
+         * Impossible. We've seen this thread running. So it should be
+         * running or at least give some results.
+         */
+        abort();
+
+    default:
+        abort();
+    }
+
+    qemu_mutex_unlock(&thr->mutex);
+
+    return res;
+}
+
+/*
+ * nbd_co_establish_connection_cancel
+ * Cancel nbd_co_establish_connection asynchronously: it will finish soon, to
+ * allow drained section to begin.
+ *
+ * If detach is true, also cleanup the state (or if thread is running, move it
+ * to CONNECT_THREAD_RUNNING_DETACHED state). s->connect_thread becomes NULL if
+ * detach is true.
+ */
+static void nbd_co_establish_connection_cancel(BlockDriverState *bs,
+                                               bool detach)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    NBDConnectThread *thr = s->connect_thread;
+    bool wake = false;
+    bool do_free = false;
+
+    qemu_mutex_lock(&thr->mutex);
+
+    if (thr->state == CONNECT_THREAD_RUNNING) {
+        /* We can cancel only in running state, when bh is not yet scheduled */
+        thr->bh_ctx = NULL;
+        if (s->wait_connect) {
+            s->wait_connect = false;
+            wake = true;
+        }
+        if (detach) {
+            thr->state = CONNECT_THREAD_RUNNING_DETACHED;
+            s->connect_thread = NULL;
+        }
+    } else if (detach) {
+        do_free = true;
+    }
+
+    qemu_mutex_unlock(&thr->mutex);
+
+    if (do_free) {
+        nbd_free_connect_thread(thr);
+        s->connect_thread = NULL;
+    }
+
+    if (wake) {
+        aio_co_wake(s->connection_co);
+    }
+}
+
 static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
 {
     int ret;
@@ -289,7 +551,7 @@ static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
         s->ioc = NULL;
     }
 
-    sioc = nbd_establish_connection(s->saddr, &local_err);
+    sioc = nbd_co_establish_connection(s->bs, &local_err);
     if (!sioc) {
         ret = -ECONNREFUSED;
         goto out;
@@ -1946,6 +2208,8 @@ static int nbd_open(BlockDriverState *bs, QDict *options, int flags,
     /* successfully connected */
     s->state = NBD_CLIENT_CONNECTED;
 
+    nbd_init_connect_thread(s);
+
     s->connection_co = qemu_coroutine_create(nbd_connection_entry, s);
     bdrv_inc_in_flight(bs);
     aio_co_schedule(bdrv_get_aio_context(bs), s->connection_co);
-- 
2.21.0

Re: [PATCH v2 for-5.1? 0/5] Fix nbd reconnect dead-locks

[Eric Blake]

On 7/27/20 1:47 PM, Vladimir Sementsov-Ogievskiy wrote:
> Hi all!
> 
> v2: it's a bit updated "[PATCH for-5.1? 0/3] Fix nbd reconnect dead-locks"
> plus completely rewritten "[PATCH for-5.1? 0/4] non-blocking connect"
> (which is now the only one patch 05)
> 
> 01: new
> 02: rebased on 01, fix (add outer "if")
> 03-04: add Eric's r-b:
> 05: new
> 
> If 05 is too big for 5.1, it's OK to take only 01-04 or less, as well as
> postponing everything to 5.2, as it's all not a degradation of 5.1
> (it's a degradation of 4.2, together with the whole reconnect feature).

I think I like where 5/5 is headed, but am not sure yet whether all 
paths are thread-safe or if there is anything we can reuse to make its 
implementation smaller.  You are right that it's probably best to defer 
that to 5.2.  In the meantime, I'll queue 1-4 for my NBD pull request 
for -rc2.

> 
> Vladimir Sementsov-Ogievskiy (5):
>    block/nbd: split nbd_establish_connection out of nbd_client_connect
>    block/nbd: allow drain during reconnect attempt
>    block/nbd: on shutdown terminate connection attempt
>    block/nbd: nbd_co_reconnect_loop(): don't sleep if drained
>    block/nbd: use non-blocking connect: fix vm hang on connect()
> 
>   block/nbd.c        | 360 +++++++++++++++++++++++++++++++++++++++++----
>   block/trace-events |   4 +-
>   2 files changed, 331 insertions(+), 33 deletions(-)
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org