Re: [PATCH v2 3/5] xhci: Add rmb() between reading event validity & event data access.
From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Date: 2011-03-29 18:56:47
On Sunday, March 27, 2011 09:52:57 pm Matt Evans wrote:
quoted hunk ↗ jump to hunk
On weakly-ordered systems, the reading of an event's content must occur after reading the event's validity. Signed-off-by: Matt Evans <redacted> --- 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