qemu/block/export
Kevin Wolf 411132c979 export/vhost-user-blk: Fix consecutive drains
The vhost-user-blk export implement AioContext switches in its drain
implementation. This means that on drain_begin, it detaches the server
from its AioContext and on drain_end, attaches it again and schedules
the server->co_trip coroutine in the updated AioContext.

However, nothing guarantees that server->co_trip is even safe to be
scheduled. Not only is it unclear that the coroutine is actually in a
state where it can be reentered externally without causing problems, but
with two consecutive drains, it is possible that the scheduled coroutine
didn't have a chance yet to run and trying to schedule an already
scheduled coroutine a second time crashes with an assertion failure.

Following the model of NBD, this commit makes the vhost-user-blk export
shut down server->co_trip during drain so that resuming the export means
creating and scheduling a new coroutine, which is always safe.

There is one exception: If the drain call didn't poll (for example, this
happens in the context of bdrv_graph_wrlock()), then the coroutine
didn't have a chance to shut down. However, in this case the AioContext
can't have changed; changing the AioContext always involves a polling
drain. So in this case we can simply assert that the AioContext is
unchanged and just leave the coroutine running or wake it up if it has
yielded to wait for the AioContext to be attached again.

Fixes: e1054cd4aa
Fixes: https://issues.redhat.com/browse/RHEL-1708
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20231127115755.22846-1-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2023-11-28 14:56:32 +01:00
..
export.c block: Mark bdrv_first_blk() and bdrv_is_root_node() GRAPH_RDLOCK 2023-10-12 16:31:33 +02:00
fuse.c aio: remove aio_disable_external() API 2023-05-30 17:37:26 +02:00
meson.build vduse-blk: Implement vduse-blk export 2022-06-24 17:07:06 +02:00
vduse-blk.c block: spelling fixes 2023-09-08 13:08:52 +03:00
vduse-blk.h vduse-blk: Implement vduse-blk export 2022-06-24 17:07:06 +02:00
vhost-user-blk-server.c export/vhost-user-blk: Fix consecutive drains 2023-11-28 14:56:32 +01:00
vhost-user-blk-server.h block: spelling fixes 2023-09-08 13:08:52 +03:00
virtio-blk-handler.c block/export: Fix graph locking in blk_get_geometry() call 2023-03-27 15:16:05 +02:00
virtio-blk-handler.h vduse-blk: Add serial option 2022-06-24 17:07:06 +02:00