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author | Greg Kurz <groug@kaod.org> | 2021-03-12 10:22:12 +0100 |
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committer | Dr. David Alan Gilbert <dgilbert@redhat.com> | 2021-03-15 20:01:55 +0000 |
commit | 5bb8327b655dbce10a91ef809acb0875dd0ee0ed (patch) | |
tree | 75f3699bd6a3243549636084979927863902afb9 /tools | |
parent | e7c6a8cf9f5c82aa152273e1c9e80d07b1b0c32c (diff) | |
download | qemu-5bb8327b655dbce10a91ef809acb0875dd0ee0ed.zip qemu-5bb8327b655dbce10a91ef809acb0875dd0ee0ed.tar.gz qemu-5bb8327b655dbce10a91ef809acb0875dd0ee0ed.tar.bz2 |
virtiofsd: Release vu_dispatch_lock when stopping queue
QEMU can stop a virtqueue by sending a VHOST_USER_GET_VRING_BASE request
to virtiofsd. As with all other vhost-user protocol messages, the thread
that runs the main event loop in virtiofsd takes the vu_dispatch lock in
write mode. This ensures that no other thread can access virtqueues or
memory tables at the same time.
In the case of VHOST_USER_GET_VRING_BASE, the main thread basically
notifies the queue thread that it should terminate and waits for its
termination:
main()
virtio_loop()
vu_dispatch_wrlock()
vu_dispatch()
vu_process_message()
vu_get_vring_base_exec()
fv_queue_cleanup_thread()
pthread_join()
Unfortunately, the queue thread ends up calling virtio_send_msg()
at some point, which itself needs to grab the lock:
fv_queue_thread()
g_list_foreach()
fv_queue_worker()
fuse_session_process_buf_int()
do_release()
lo_release()
fuse_reply_err()
send_reply()
send_reply_iov()
fuse_send_reply_iov_nofree()
fuse_send_msg()
virtio_send_msg()
vu_dispatch_rdlock() <-- Deadlock !
Simply have the main thread to release the lock before going to
sleep and take it back afterwards. A very similar patch was already
sent by Vivek Goyal sometime back:
https://listman.redhat.com/archives/virtio-fs/2021-January/msg00073.html
The only difference here is that this done in fv_queue_set_started()
because fv_queue_cleanup_thread() can also be called from virtio_loop()
without the lock being held.
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20210312092212.782255-8-groug@kaod.org>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Diffstat (limited to 'tools')
-rw-r--r-- | tools/virtiofsd/fuse_virtio.c | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c index 523ee64..3e13997 100644 --- a/tools/virtiofsd/fuse_virtio.c +++ b/tools/virtiofsd/fuse_virtio.c @@ -792,7 +792,13 @@ static void fv_queue_set_started(VuDev *dev, int qidx, bool started) assert(0); } } else { + /* + * Temporarily drop write-lock taken in virtio_loop() so that + * the queue thread doesn't block in virtio_send_msg(). + */ + vu_dispatch_unlock(vud); fv_queue_cleanup_thread(vud, qidx); + vu_dispatch_wrlock(vud); } } |