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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>
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@ -792,7 +792,13 @@ static void fv_queue_set_started(VuDev *dev, int qidx, bool started)
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assert(0);
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}
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} else {
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/*
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* Temporarily drop write-lock taken in virtio_loop() so that
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* the queue thread doesn't block in virtio_send_msg().
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*/
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vu_dispatch_unlock(vud);
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fv_queue_cleanup_thread(vud, qidx);
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vu_dispatch_wrlock(vud);
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}
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}
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