Re: [patch 4/4] UHCI: add missing memory barriers

Alan Stern
2005-12-13T03:32:39Z

On Mon, 12 Dec 2005, Jeff Garzik wrote:

> Greg Kroah-Hartman wrote:
> > From: Alan Stern <stern@rowland.harvard.edu>
> >
> > This patch (as617) adds a couple of memory barriers that Ben H. forgot in
> > his recent suspend/resume fix.
> >
> > Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
> >
> > ---
> > drivers/usb/host/uhci-hcd.c | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > --- greg-2.6.orig/drivers/usb/host/uhci-hcd.c
> > +++ greg-2.6/drivers/usb/host/uhci-hcd.c
> > @@ -717,6 +717,7 @@ static int uhci_suspend(struct usb_hcd *
> > * at the source, so we must turn off PIRQ.
> > */
> > pci_write_config_word(to_pci_dev(uhci_dev(uhci)), USBLEGSUP, 0);
> > + mb();
> > clear_bit(HCD_FLAG_HW_ACCESSIBLE, &hcd->flags);
> > uhci->hc_inaccessible = 1;
> > hcd->poll_rh = 0;
> > @@ -738,6 +739,7 @@ static int uhci_resume(struct usb_hcd *h
> > * really don't want to keep a stale HCD_FLAG_HW_ACCESSIBLE=0
> > */
> > set_bit(HCD_FLAG_HW_ACCESSIBLE, &hcd->flags);
> > + mb();
>
> Are these just guesses, or what?

There's no need for sarcasm. No, they are not guesses. Ask Ben
Herrenschmidt if you don't believe me.

> Why not smp_mb__before_clear_bit() or smp_mb__after_clear_bit() ?

Because the code needs to synchronize not with another CPU, but with a USB
host controller. Those barriers are necessary even on a UP system.

By the way, what's the idea with this proliferation of little
not-all-that-helpful routines, like smp_mb__before_clear_bit()? Are there
architectures on which

smp_mb__before_clear_bit(...);

is significantly superior to

smp_mb();
clear_bit(...);

? (It's certainly not easier to type.) Is this difference worth noting,
considering how infrequently clear_bit() gets used?

Alan Stern