linux-kernel - Re: [PATCH] 9p/virtio: add a read barrier in p9_virtio_zc

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Date:   Tue, 13 Dec 2022 13:03:52 +0900
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Christian Schoenebeck <linux_oss@...debyte.com>
Cc:     v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        Marco Elver <elver@...gle.com>
Subject: Re: [PATCH] 9p/virtio: add a read barrier in p9_virtio_zc_request

Christian Schoenebeck wrote on Mon, Dec 12, 2022 at 02:35:39PM +0100:
> > 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();
> > +
> 
> Oh, I had p9_client_zc_rpc() for this in mind, but I see why you chose this
> place in p9_virtio_zc_request() instead. LGTM

Yes, we access req data here so as much as it'd make more sense to keep
it symetrical in p9_client_zc_rpc (like p9_client_rpc) I think we need
it here.

> I also made some tests to check whether this barrier would hurt performance,
> but I measured no difference. So this should be good to go:

Thanks!
The assembly generated with the barrier is actually slightly shorter for
x86_64, but it's hard to tell the actual performance impact....

> Reviewed-by: Christian Schoenebeck <linux_oss@...debyte.com>

Cheers, I've queued this patch as well: let's make that this merge
windows' batch unless a problem comes up.

-- 
Dominique