[PATCH] 9p/virtio: add a read barrier in p9_virtio_zc_request

[Dominique Martinet <asmadeus@codewreck.org>]

The request receiving thread writes into request then marks the request
valid in p9_client_cb by setting status after a write barrier.

p9_virtio_zc_request must like p9_client_rpc issue a read barrier after
getting notified of the new request status before reading other request
files.

(This has been noticed while fixing the usage of READ/WRITE_ONCE macros
for request status)

Link: https://lkml.kernel.org/r/167052961.MU3OA6Uzks@silver
Reported-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
---
 net/9p/trans_virtio.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 3c27ffb781e3..98425c63b3c3 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -533,6 +533,12 @@ p9_virtio_zc_request(struct p9_client *client, struct p9_req_t *req,
 	p9_debug(P9_DEBUG_TRANS, "virtio request kicked\n");
 	err = wait_event_killable(req->wq,
 			          READ_ONCE(req->status) >= REQ_STATUS_RCVD);
+
+	/* Make sure our req is coherent with regard to updates in other
+	 * threads - echoes to wmb() in the callback like p9_client_rpc
+	 */
+	smp_rmb();
+
 	// RERROR needs reply (== error string) in static data
 	if (READ_ONCE(req->status) == REQ_STATUS_RCVD &&
 	    unlikely(req->rc.sdata[4] == P9_RERROR))
-- 
2.38.1