Re: [PATCH 36/78] USB: add power/level sysfs attribute

On Fri, 27 Apr 2007, Oliver Neukum wrote:

> Am Freitag, 27. April 2007 20:55 schrieb Greg Kroah-Hartman:
> > +       } else if (len == sizeof suspend_string - 1 &&
> > +                       strncmp(buf, suspend_string, len) == 0) {
> > +               udev->autosuspend_disabled = 0;
> > +               udev->autoresume_disabled = 1;
> > +               rc = usb_external_suspend_device(udev, PMSG_SUSPEND);
> > +
> 
> Hi,
> 
> this introduces the possibility of a suspension while user space is
> doing IO.

In other words, one user can force a device to suspend while another user 
is trying to access the device.  True enough.  The second user will get 
I/O errors.

This is why sysfs files need write-access restrictions.

> Now I know that I was in favor of doing this. But on second
> thought, it is problematic. It seems to me we should have a way
> to generically block io. This is not solvable in the suspend method,
> as we can't tell this apart from STR. We'd need locks between
> suspend and all io methods.
> 
> What's to be done?

Why bother to block I/O?  Let it fail.  It's no worse than what happens 
when you unplug a USB device.

In general, I/O shouldn't be blocked at the device driver level.  It 
should be blocked higher up, in a class device or an I/O queue manager -- 
if it is to be blocked at all.

Alan Stern


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