Subject: Re: [PATCH v2 3/5] xhci: Add rmb() between reading event validity & event data access.

From: Dmitry Torokhov <[email-masked]>
Date: Tue, 29 Mar 2011 11:56:47 -0700

On Sunday, March 27, 2011 09:52:57 pm Matt Evans wrote:
> On weakly-ordered systems, the reading of an event's content must occur
> after reading the event's validity.
> 
> Signed-off-by: Matt Evans <[email-masked]>
> ---
> Segher, thanks for the comment; explanation added.
> 
>  drivers/usb/host/xhci-ring.c |    5 +++++
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
> index 45f3b77..d6aa880 100644
> --- a/drivers/usb/host/xhci-ring.c
> +++ b/drivers/usb/host/xhci-ring.c
> @@ -2152,6 +2152,11 @@ static void xhci_handle_event(struct xhci_hcd
> *xhci) }
>  	xhci_dbg(xhci, "%s - OS owns TRB\n", __func__);
> 
> +	/*
> +	 * Barrier between reading the TRB_CYCLE (valid) flag above and any
> +	 * speculative reads of the event's flags/data below.
> +	 */
> +	rmb();
>  	/* FIXME: Handle more event types. */
>  	switch ((le32_to_cpu(event->event_cmd.flags) & TRB_TYPE_BITMASK)) {

Isn't it the same memory that is being read the first time around? How 
reordering could happen here?

Thanks.

-- 
Dmitry