Re: [PATCH] block/export: Fix vhost-user-blk shutdown with requests in flight

[Stefan Hajnoczi]

On Tue, Jan 25, 2022 at 04:14:35PM +0100, Kevin Wolf wrote:
> The vhost-user-blk export runs requests asynchronously in their own
> coroutine. When the vhost connection goes away and we want to stop the
> vhost-user server, we need to wait for these coroutines to stop before
> we can unmap the shared memory. Otherwise, they would still access the
> unmapped memory and crash.
> 
> This introduces a refcount to VuServer which is increased when spawning
> a new request coroutine and decreased before the coroutine exits. The
> memory is only unmapped when the refcount reaches zero.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  include/qemu/vhost-user-server.h     |  5 +++++
>  block/export/vhost-user-blk-server.c |  5 +++++
>  util/vhost-user-server.c             | 22 ++++++++++++++++++++++
>  3 files changed, 32 insertions(+)
> 
> diff --git a/include/qemu/vhost-user-server.h 
> b/include/qemu/vhost-user-server.h
> index 121ea1dedf..cd43193b80 100644
> --- a/include/qemu/vhost-user-server.h
> +++ b/include/qemu/vhost-user-server.h
> @@ -42,6 +42,8 @@ typedef struct {
>      const VuDevIface *vu_iface;
>  
>      /* Protected by ctx lock */
> +    unsigned int refcount;
> +    bool wait_idle;
>      VuDev vu_dev;
>      QIOChannel *ioc; /* The I/O channel with the client */
>      QIOChannelSocket *sioc; /* The underlying data channel with the client */
> @@ -59,6 +61,9 @@ bool vhost_user_server_start(VuServer *server,
>  
>  void vhost_user_server_stop(VuServer *server);
>  
> +void vhost_user_server_ref(VuServer *server);
> +void vhost_user_server_unref(VuServer *server);
> +
>  void vhost_user_server_attach_aio_context(VuServer *server, AioContext *ctx);
>  void vhost_user_server_detach_aio_context(VuServer *server);
>  
> diff --git a/block/export/vhost-user-blk-server.c 
> b/block/export/vhost-user-blk-server.c
> index 1862563336..a129204c44 100644
> --- a/block/export/vhost-user-blk-server.c
> +++ b/block/export/vhost-user-blk-server.c
> @@ -172,6 +172,7 @@ vu_blk_discard_write_zeroes(VuBlkExport *vexp, struct 
> iovec *iov,
>      return VIRTIO_BLK_S_IOERR;
>  }
>  
> +/* Called with server refcount increased, must decrease before returning */
>  static void coroutine_fn vu_blk_virtio_process_req(void *opaque)
>  {
>      VuBlkReq *req = opaque;
> @@ -286,10 +287,12 @@ static void coroutine_fn vu_blk_virtio_process_req(void 
> *opaque)
>      }
>  
>      vu_blk_req_complete(req);
> +    vhost_user_server_unref(server);
>      return;
>  
>  err:
>      free(req);
> +    vhost_user_server_unref(server);
>  }
>  
>  static void vu_blk_process_vq(VuDev *vu_dev, int idx)
> @@ -310,6 +313,8 @@ static void vu_blk_process_vq(VuDev *vu_dev, int idx)
>  
>          Coroutine *co =
>              qemu_coroutine_create(vu_blk_virtio_process_req, req);
> +
> +        vhost_user_server_ref(server);
>          qemu_coroutine_enter(co);

Why not increment inside vu_blk_virtio_process_req()? My understanding
is the coroutine is entered immediately so there is no race that needs
to be protected against by incrementing the refcount early.

>      }
>  }
> diff --git a/util/vhost-user-server.c b/util/vhost-user-server.c
> index f68287e811..f66fbba710 100644
> --- a/util/vhost-user-server.c
> +++ b/util/vhost-user-server.c
> @@ -74,6 +74,20 @@ static void panic_cb(VuDev *vu_dev, const char *buf)
>      error_report("vu_panic: %s", buf);
>  }
>  
> +void vhost_user_server_ref(VuServer *server)
> +{
> +    assert(!server->wait_idle);
> +    server->refcount++;
> +}
> +
> +void vhost_user_server_unref(VuServer *server)
> +{
> +    server->refcount--;
> +    if (server->wait_idle && !server->refcount) {
> +        aio_co_wake(server->co_trip);
> +    }
> +}
> +
>  static bool coroutine_fn
>  vu_message_read(VuDev *vu_dev, int conn_fd, VhostUserMsg *vmsg)
>  {
> @@ -177,6 +191,14 @@ static coroutine_fn void vu_client_trip(void *opaque)
>          /* Keep running */
>      }
>  
> +    if (server->refcount) {
> +        /* Wait for requests to complete before we can unmap the memory */
> +        server->wait_idle = true;
> +        qemu_coroutine_yield();
> +        server->wait_idle = false;
> +    }
> +    assert(server->refcount == 0);
> +
>      vu_deinit(vu_dev);
>  
>      /* vu_deinit() should have called remove_watch() */
> -- 
> 2.31.1
>

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