diff mbox series

[v2,6/6] nbd/server: introduce NBDClient->lock to protect fields

Message ID 20231221153548.1752005-7-stefanha@redhat.com
State New
Headers show
Series qemu-iotests fixes for Kevin's block tree | expand

Commit Message

Stefan Hajnoczi Dec. 21, 2023, 3:35 p.m. UTC 
NBDClient has a number of fields that are accessed by both the export
AioContext and the main loop thread. When the AioContext lock is removed
these fields will need another form of protection.

Add NBDClient->lock and protect fields that are accessed by both
threads. Also add assertions where possible and otherwise add doc
comments stating assumptions about which thread and lock holding.

Note this patch moves the client->recv_coroutine assertion from
nbd_co_receive_request() to nbd_trip() where client->lock is held.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 nbd/server.c | 141 +++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 108 insertions(+), 33 deletions(-)

Comments

Kevin Wolf Dec. 21, 2023, 5:38 p.m. UTC | #1 
Am 21.12.2023 um 16:35 hat Stefan Hajnoczi geschrieben:
> NBDClient has a number of fields that are accessed by both the export
> AioContext and the main loop thread. When the AioContext lock is removed
> these fields will need another form of protection.
> 
> Add NBDClient->lock and protect fields that are accessed by both
> threads. Also add assertions where possible and otherwise add doc
> comments stating assumptions about which thread and lock holding.
> 
> Note this patch moves the client->recv_coroutine assertion from
> nbd_co_receive_request() to nbd_trip() where client->lock is held.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

> +/* Runs in export AioContext */
> +static void nbd_wake_read_bh(void *opaque)
> +{
> +    NBDClient *client = opaque;
> +    qio_channel_wake_read(client->ioc);
> +}
> +
>  static bool nbd_drained_poll(void *opaque)
>  {
>      NBDExport *exp = opaque;
>      NBDClient *client;
>  
> +    assert(qemu_in_main_thread());
> +
>      QTAILQ_FOREACH(client, &exp->clients, next) {
> -        if (client->nb_requests != 0) {
> -            /*
> -             * If there's a coroutine waiting for a request on nbd_read_eof()
> -             * enter it here so we don't depend on the client to wake it up.
> -             */
> -            if (client->recv_coroutine != NULL && client->read_yielding) {
> -                qio_channel_wake_read(client->ioc);
> +        WITH_QEMU_LOCK_GUARD(&client->lock) {
> +            if (client->nb_requests != 0) {
> +                /*
> +                 * If there's a coroutine waiting for a request on nbd_read_eof()
> +                 * enter it here so we don't depend on the client to wake it up.
> +                 *
> +                 * Schedule a BH in the export AioContext to avoid missing the
> +                 * wake up due to the race between qio_channel_wake_read() and
> +                 * qio_channel_yield().
> +                 */
> +                if (client->recv_coroutine != NULL && client->read_yielding) {
> +                    aio_bh_schedule_oneshot(nbd_export_aio_context(client->exp),
> +                                            nbd_wake_read_bh, client);
> +                }

Doesn't the condition have to move inside the BH to avoid the race?

Checking client->recv_coroutine != NULL could work here because I don't
think it can go from NULL to something while we're quiescing, but
client->read_yielding can still change until the BH runs and we know
that the nbd_co_trip() coroutine has yielded. It seems easiest to just
move the whole condition to the BH.

> +                return true;
>              }
> -
> -            return true;
>          }
>      }

The rest looks good to me.

Kevin
Stefan Hajnoczi Dec. 21, 2023, 7:21 p.m. UTC | #2 
On Thu, Dec 21, 2023 at 06:38:16PM +0100, Kevin Wolf wrote:
> Am 21.12.2023 um 16:35 hat Stefan Hajnoczi geschrieben:
> > NBDClient has a number of fields that are accessed by both the export
> > AioContext and the main loop thread. When the AioContext lock is removed
> > these fields will need another form of protection.
> > 
> > Add NBDClient->lock and protect fields that are accessed by both
> > threads. Also add assertions where possible and otherwise add doc
> > comments stating assumptions about which thread and lock holding.
> > 
> > Note this patch moves the client->recv_coroutine assertion from
> > nbd_co_receive_request() to nbd_trip() where client->lock is held.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> 
> > +/* Runs in export AioContext */
> > +static void nbd_wake_read_bh(void *opaque)
> > +{
> > +    NBDClient *client = opaque;
> > +    qio_channel_wake_read(client->ioc);
> > +}
> > +
> >  static bool nbd_drained_poll(void *opaque)
> >  {
> >      NBDExport *exp = opaque;
> >      NBDClient *client;
> >  
> > +    assert(qemu_in_main_thread());
> > +
> >      QTAILQ_FOREACH(client, &exp->clients, next) {
> > -        if (client->nb_requests != 0) {
> > -            /*
> > -             * If there's a coroutine waiting for a request on nbd_read_eof()
> > -             * enter it here so we don't depend on the client to wake it up.
> > -             */
> > -            if (client->recv_coroutine != NULL && client->read_yielding) {
> > -                qio_channel_wake_read(client->ioc);
> > +        WITH_QEMU_LOCK_GUARD(&client->lock) {
> > +            if (client->nb_requests != 0) {
> > +                /*
> > +                 * If there's a coroutine waiting for a request on nbd_read_eof()
> > +                 * enter it here so we don't depend on the client to wake it up.
> > +                 *
> > +                 * Schedule a BH in the export AioContext to avoid missing the
> > +                 * wake up due to the race between qio_channel_wake_read() and
> > +                 * qio_channel_yield().
> > +                 */
> > +                if (client->recv_coroutine != NULL && client->read_yielding) {
> > +                    aio_bh_schedule_oneshot(nbd_export_aio_context(client->exp),
> > +                                            nbd_wake_read_bh, client);
> > +                }
> 
> Doesn't the condition have to move inside the BH to avoid the race?
> 
> Checking client->recv_coroutine != NULL could work here because I don't
> think it can go from NULL to something while we're quiescing, but
> client->read_yielding can still change until the BH runs and we know
> that the nbd_co_trip() coroutine has yielded. It seems easiest to just
> move the whole condition to the BH.

I will add aio_wait_kick() into nbd_read_eof() immediately after setting
client->read_yielding. That way nbd_drained_poll() re-runs after
client->read_yielding is set to true.

Stefan
diff mbox series

Patch

diff --git a/nbd/server.c b/nbd/server.c
index e91e2e0903..5b08aae535 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -125,23 +125,25 @@  struct NBDClient {
     int refcount; /* atomic */
     void (*close_fn)(NBDClient *client, bool negotiated);
 
+    QemuMutex lock;
+
     NBDExport *exp;
     QCryptoTLSCreds *tlscreds;
     char *tlsauthz;
     QIOChannelSocket *sioc; /* The underlying data channel */
     QIOChannel *ioc; /* The current I/O channel which may differ (eg TLS) */
 
-    Coroutine *recv_coroutine;
+    Coroutine *recv_coroutine; /* protected by lock */
 
     CoMutex send_lock;
     Coroutine *send_coroutine;
 
-    bool read_yielding;
-    bool quiescing;
+    bool read_yielding; /* protected by lock */
+    bool quiescing; /* protected by lock */
 
     QTAILQ_ENTRY(NBDClient) next;
-    int nb_requests;
-    bool closing;
+    int nb_requests; /* protected by lock */
+    bool closing; /* protected by lock */
 
     uint32_t check_align; /* If non-zero, check for aligned client requests */
 
@@ -1415,11 +1417,15 @@  nbd_read_eof(NBDClient *client, void *buffer, size_t size, Error **errp)
 
         len = qio_channel_readv(client->ioc, &iov, 1, errp);
         if (len == QIO_CHANNEL_ERR_BLOCK) {
-            client->read_yielding = true;
+            WITH_QEMU_LOCK_GUARD(&client->lock) {
+                client->read_yielding = true;
+            }
             qio_channel_yield(client->ioc, G_IO_IN);
-            client->read_yielding = false;
-            if (client->quiescing) {
-                return -EAGAIN;
+            WITH_QEMU_LOCK_GUARD(&client->lock) {
+                client->read_yielding = false;
+                if (client->quiescing) {
+                    return -EAGAIN;
+                }
             }
             continue;
         } else if (len < 0) {
@@ -1528,6 +1534,7 @@  void nbd_client_put(NBDClient *client)
             blk_exp_unref(&client->exp->common);
         }
         g_free(client->contexts.bitmaps);
+        qemu_mutex_destroy(&client->lock);
         g_free(client);
     }
 }
@@ -1561,11 +1568,13 @@  static void client_close(NBDClient *client, bool negotiated)
 {
     assert(qemu_in_main_thread());
 
-    if (client->closing) {
-        return;
-    }
+    WITH_QEMU_LOCK_GUARD(&client->lock) {
+        if (client->closing) {
+            return;
+        }
 
-    client->closing = true;
+        client->closing = true;
+    }
 
     /* Force requests to finish.  They will drop their own references,
      * then we'll close the socket and free the NBDClient.
@@ -1579,6 +1588,7 @@  static void client_close(NBDClient *client, bool negotiated)
     }
 }
 
+/* Runs in export AioContext with client->lock held */
 static NBDRequestData *nbd_request_get(NBDClient *client)
 {
     NBDRequestData *req;
@@ -1591,6 +1601,7 @@  static NBDRequestData *nbd_request_get(NBDClient *client)
     return req;
 }
 
+/* Runs in export AioContext with client->lock held */
 static void nbd_request_put(NBDRequestData *req)
 {
     NBDClient *client = req->client;
@@ -1614,14 +1625,18 @@  static void blk_aio_attached(AioContext *ctx, void *opaque)
     NBDExport *exp = opaque;
     NBDClient *client;
 
+    assert(qemu_in_main_thread());
+
     trace_nbd_blk_aio_attached(exp->name, ctx);
 
     exp->common.ctx = ctx;
 
     QTAILQ_FOREACH(client, &exp->clients, next) {
-        assert(client->nb_requests == 0);
-        assert(client->recv_coroutine == NULL);
-        assert(client->send_coroutine == NULL);
+        WITH_QEMU_LOCK_GUARD(&client->lock) {
+            assert(client->nb_requests == 0);
+            assert(client->recv_coroutine == NULL);
+            assert(client->send_coroutine == NULL);
+        }
     }
 }
 
@@ -1629,6 +1644,8 @@  static void blk_aio_detach(void *opaque)
 {
     NBDExport *exp = opaque;
 
+    assert(qemu_in_main_thread());
+
     trace_nbd_blk_aio_detach(exp->name, exp->common.ctx);
 
     exp->common.ctx = NULL;
@@ -1639,8 +1656,12 @@  static void nbd_drained_begin(void *opaque)
     NBDExport *exp = opaque;
     NBDClient *client;
 
+    assert(qemu_in_main_thread());
+
     QTAILQ_FOREACH(client, &exp->clients, next) {
-        client->quiescing = true;
+        WITH_QEMU_LOCK_GUARD(&client->lock) {
+            client->quiescing = true;
+        }
     }
 }
 
@@ -1649,28 +1670,48 @@  static void nbd_drained_end(void *opaque)
     NBDExport *exp = opaque;
     NBDClient *client;
 
+    assert(qemu_in_main_thread());
+
     QTAILQ_FOREACH(client, &exp->clients, next) {
-        client->quiescing = false;
-        nbd_client_receive_next_request(client);
+        WITH_QEMU_LOCK_GUARD(&client->lock) {
+            client->quiescing = false;
+            nbd_client_receive_next_request(client);
+        }
     }
 }
 
+/* Runs in export AioContext */
+static void nbd_wake_read_bh(void *opaque)
+{
+    NBDClient *client = opaque;
+    qio_channel_wake_read(client->ioc);
+}
+
 static bool nbd_drained_poll(void *opaque)
 {
     NBDExport *exp = opaque;
     NBDClient *client;
 
+    assert(qemu_in_main_thread());
+
     QTAILQ_FOREACH(client, &exp->clients, next) {
-        if (client->nb_requests != 0) {
-            /*
-             * If there's a coroutine waiting for a request on nbd_read_eof()
-             * enter it here so we don't depend on the client to wake it up.
-             */
-            if (client->recv_coroutine != NULL && client->read_yielding) {
-                qio_channel_wake_read(client->ioc);
+        WITH_QEMU_LOCK_GUARD(&client->lock) {
+            if (client->nb_requests != 0) {
+                /*
+                 * If there's a coroutine waiting for a request on nbd_read_eof()
+                 * enter it here so we don't depend on the client to wake it up.
+                 *
+                 * Schedule a BH in the export AioContext to avoid missing the
+                 * wake up due to the race between qio_channel_wake_read() and
+                 * qio_channel_yield().
+                 */
+                if (client->recv_coroutine != NULL && client->read_yielding) {
+                    aio_bh_schedule_oneshot(nbd_export_aio_context(client->exp),
+                                            nbd_wake_read_bh, client);
+                }
+
+                return true;
             }
-
-            return true;
         }
     }
 
@@ -1681,6 +1722,8 @@  static void nbd_eject_notifier(Notifier *n, void *data)
 {
     NBDExport *exp = container_of(n, NBDExport, eject_notifier);
 
+    assert(qemu_in_main_thread());
+
     blk_exp_request_shutdown(&exp->common);
 }
 
@@ -2566,7 +2609,6 @@  static int coroutine_fn nbd_co_receive_request(NBDRequestData *req,
     int ret;
 
     g_assert(qemu_in_coroutine());
-    assert(client->recv_coroutine == qemu_coroutine_self());
     ret = nbd_receive_request(client, request, errp);
     if (ret < 0) {
         return ret;
@@ -2975,6 +3017,9 @@  static coroutine_fn void nbd_trip(void *opaque)
      */
 
     trace_nbd_trip();
+
+    qemu_mutex_lock(&client->lock);
+
     if (client->closing) {
         goto done;
     }
@@ -2990,7 +3035,21 @@  static coroutine_fn void nbd_trip(void *opaque)
     }
 
     req = nbd_request_get(client);
-    ret = nbd_co_receive_request(req, &request, &local_err);
+
+    /*
+     * nbd_co_receive_request() returns -EAGAIN when nbd_drained_begin() has
+     * set client->quiescing but by the time we get back nbd_drained_end() may
+     * have already cleared client->quiescing. In that case we try again
+     * because nothing else will spawn an nbd_trip() coroutine until we set
+     * client->recv_coroutine = NULL further down.
+     */
+    do {
+        assert(client->recv_coroutine == qemu_coroutine_self());
+        qemu_mutex_unlock(&client->lock);
+        ret = nbd_co_receive_request(req, &request, &local_err);
+        qemu_mutex_lock(&client->lock);
+    } while (ret == -EAGAIN && !client->quiescing);
+
     client->recv_coroutine = NULL;
 
     if (client->closing) {
@@ -3002,15 +3061,16 @@  static coroutine_fn void nbd_trip(void *opaque)
     }
 
     if (ret == -EAGAIN) {
-        assert(client->quiescing);
         goto done;
     }
 
     nbd_client_receive_next_request(client);
+
     if (ret == -EIO) {
         goto disconnect;
     }
 
+    qemu_mutex_unlock(&client->lock);
     qio_channel_set_cork(client->ioc, true);
 
     if (ret < 0) {
@@ -3030,6 +3090,10 @@  static coroutine_fn void nbd_trip(void *opaque)
         g_free(request.contexts->bitmaps);
         g_free(request.contexts);
     }
+
+    qio_channel_set_cork(client->ioc, false);
+    qemu_mutex_lock(&client->lock);
+
     if (ret < 0) {
         error_prepend(&local_err, "Failed to send reply: ");
         goto disconnect;
@@ -3044,11 +3108,13 @@  static coroutine_fn void nbd_trip(void *opaque)
         goto disconnect;
     }
 
-    qio_channel_set_cork(client->ioc, false);
 done:
     if (req) {
         nbd_request_put(req);
     }
+
+    qemu_mutex_unlock(&client->lock);
+
     if (!nbd_client_put_nonzero(client)) {
         aio_co_reschedule_self(qemu_get_aio_context());
         nbd_client_put(client);
@@ -3059,13 +3125,19 @@  disconnect:
     if (local_err) {
         error_reportf_err(local_err, "Disconnect client, due to: ");
     }
+
     nbd_request_put(req);
+    qemu_mutex_unlock(&client->lock);
 
     aio_co_reschedule_self(qemu_get_aio_context());
     client_close(client, true);
     nbd_client_put(client);
 }
 
+/*
+ * Runs in export AioContext and main loop thread. Caller must hold
+ * client->lock.
+ */
 static void nbd_client_receive_next_request(NBDClient *client)
 {
     if (!client->recv_coroutine && client->nb_requests < MAX_NBD_REQUESTS &&
@@ -3091,7 +3163,9 @@  static coroutine_fn void nbd_co_client_start(void *opaque)
         return;
     }
 
-    nbd_client_receive_next_request(client);
+    WITH_QEMU_LOCK_GUARD(&client->lock) {
+        nbd_client_receive_next_request(client);
+    }
 }
 
 /*
@@ -3108,6 +3182,7 @@  void nbd_client_new(QIOChannelSocket *sioc,
     Coroutine *co;
 
     client = g_new0(NBDClient, 1);
+    qemu_mutex_init(&client->lock);
     client->refcount = 1;
     client->tlscreds = tlscreds;
     if (tlscreds) {