Messages in this thread | | | Date | Mon, 12 Dec 2005 22:32:37 -0500 (EST) | From | Alan Stern <> | Subject | Re: [patch 4/4] UHCI: add missing memory barriers |
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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
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