xen-bus/block: explicitly assign event channels to an AioContext

It is not safe to close an event channel from the QEMU main thread when
that channel's poller is running in IOThread context.

This patch adds a new xen_device_set_event_channel_context() function
to explicitly assign the channel AioContext, and modifies
xen_device_bind_event_channel() to initially assign the channel's poller
to the QEMU main thread context. The code in xen-block's dataplane is
then modified to assign the channel to IOThread context during
xen_block_dataplane_start() and de-assign it during in
xen_block_dataplane_stop(), such that the channel is always assigned
back to main thread context before it is closed. aio_set_fd_handler()
already deals with all the necessary synchronization when moving an fd
between AioContext-s so no extra code is needed to manage this.

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20191216143451.19024-1-pdurrant@amazon.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
(cherry picked from commit 32d0b7be68216f130dc962533c6fc2f9ad848a8b)

diff --git a/hw/block/dataplane/xen-block.c b/hw/block/dataplane/xen-block.c
index 0f200c5fb0d6139b7a72f60423138927e0c46d90..18ba4ac897316b6926a18f1747d3546c4ab103b1 100644 (file)
--- a/hw/block/dataplane/xen-block.c
+++ b/hw/block/dataplane/xen-block.c
@@ -685,12 +685,24 @@ void xen_block_dataplane_stop(XenBlockDataPlane *dataplane)
         return;
     }
 
+    xendev = dataplane->xendev;
+
     aio_context_acquire(dataplane->ctx);
+    if (dataplane->event_channel) {
+        /* Only reason for failure is a NULL channel */
+        xen_device_set_event_channel_context(xendev, dataplane->event_channel,
+                                             qemu_get_aio_context(),
+                                             &error_abort);
+    }
     /* Xen doesn't have multiple users for nodes, so this can't fail */
     blk_set_aio_context(dataplane->blk, qemu_get_aio_context(), &error_abort);
     aio_context_release(dataplane->ctx);
 
-    xendev = dataplane->xendev;
+    /*
+     * Now that the context has been moved onto the main thread, cancel
+     * further processing.
+     */
+    qemu_bh_cancel(dataplane->bh);
 
     if (dataplane->event_channel) {
         Error *local_err = NULL;
@@ -807,7 +819,7 @@ void xen_block_dataplane_start(XenBlockDataPlane *dataplane,
     }
 
     dataplane->event_channel =
-        xen_device_bind_event_channel(xendev, dataplane->ctx, event_channel,
+        xen_device_bind_event_channel(xendev, event_channel,
                                       xen_block_dataplane_event, dataplane,
                                       &local_err);
     if (local_err) {
@@ -818,7 +830,11 @@ void xen_block_dataplane_start(XenBlockDataPlane *dataplane,
     aio_context_acquire(dataplane->ctx);
     /* If other users keep the BlockBackend in the iothread, that's ok */
     blk_set_aio_context(dataplane->blk, dataplane->ctx, NULL);
+    /* Only reason for failure is a NULL channel */
+    xen_device_set_event_channel_context(xendev, dataplane->event_channel,
+                                         dataplane->ctx, &error_abort);
     aio_context_release(dataplane->ctx);
+
     return;
 
 stop:

diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index 63469e0cf9a7b2fc15d0c7c943bb840a3eeadb57..cf79c5e3a86b83d45c8c00a9c0369679066e91ab 100644 (file)
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -1089,8 +1089,26 @@ static void xen_device_event(void *opaque)
     }
 }
 
+void xen_device_set_event_channel_context(XenDevice *xendev,
+                                          XenEventChannel *channel,
+                                          AioContext *ctx,
+                                          Error **errp)
+{
+    if (!channel) {
+        error_setg(errp, "bad channel");
+        return;
+    }
+
+    if (channel->ctx)
+        aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
+                           NULL, NULL, NULL, NULL);
+
+    channel->ctx = ctx;
+    aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
+                       xen_device_event, NULL, xen_device_poll, channel);
+}
+
 XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
-                                               AioContext *ctx,
                                                unsigned int port,
                                                XenEventHandler handler,
                                                void *opaque, Error **errp)
@@ -1116,9 +1134,10 @@ XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
     channel->handler = handler;
     channel->opaque = opaque;
 
-    channel->ctx = ctx;
-    aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
-                       xen_device_event, NULL, xen_device_poll, channel);
+    /* Only reason for failure is a NULL channel */
+    xen_device_set_event_channel_context(xendev, channel,
+                                         qemu_get_aio_context(),
+                                         &error_abort);
 
     QLIST_INSERT_HEAD(&xendev->event_channels, channel, list);
 

diff --git a/include/hw/xen/xen-bus.h b/include/hw/xen/xen-bus.h
index 3d5532258df703db9e1d9681db76b482d2b369cc..c18c1372af38264d4b9066a151ec1e28b8c48b5c 100644 (file)
--- a/include/hw/xen/xen-bus.h
+++ b/include/hw/xen/xen-bus.h
@@ -128,10 +128,13 @@ void xen_device_copy_grant_refs(XenDevice *xendev, bool to_domain,
 typedef bool (*XenEventHandler)(void *opaque);
 
 XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
-                                               AioContext *ctx,
                                                unsigned int port,
                                                XenEventHandler handler,
                                                void *opaque, Error **errp);
+void xen_device_set_event_channel_context(XenDevice *xendev,
+                                          XenEventChannel *channel,
+                                          AioContext *ctx,
+                                          Error **errp);
 void xen_device_notify_event_channel(XenDevice *xendev,
                                      XenEventChannel *channel,
                                      Error **errp);