Re: [PATCH 10/22] gpiolib: cdev: fix minor race in GET_LINEINFO_WATCH

[Kent Gibson]

On Wed, Jun 24, 2020 at 05:46:33PM +0300, Andy Shevchenko wrote:
> On Tue, Jun 23, 2020 at 7:03 AM Kent Gibson <warthog618@xxxxxxxxx> wrote:
> >
> > Merge separate usage of test_bit/set_bit into test_and_set_bit to remove
> > the possibility of a race between the test and set.
> >
> > Similarly test_bit and clear_bit.
> >
> > In the existing code it is possible for two threads to race past the
> > test_bit and then set or clear the watch bit, and neither return EBUSY.
> 
> I stumbled over this myself, but...
> 
> > -               if (test_bit(hwgpio, gcdev->watched_lines))
> > +               if (test_and_set_bit(hwgpio, gcdev->watched_lines))
> >                         return -EBUSY;
> >
> >                 gpio_desc_to_lineinfo(desc, &lineinfo);
> > @@ -897,7 +897,6 @@ static long gpio_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> >                 if (copy_to_user(ip, &lineinfo, sizeof(lineinfo)))
> >                         return -EFAULT;
> >
> > -               set_bit(hwgpio, gcdev->watched_lines);
> >                 return 0;
> 
> ...I think it's not an equivalent despite races involved. If you set
> bit and return error code, you will have the wrong state.
> 

Not quite sure what you mean.  There is only an error if the bit is
already set, so you've changed nothing.

And the watched state is not part of the lineinfo, so the state returned is
the same either way.

Cheers,
Kent.