Re: [PATCH RESEND] HID: multitouch: Add memory barriers

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Aug 17, 2022 at 04:14:20PM +0000, Andri Yngvason wrote:
> Hi Greg,
> 
> mið., 17. ágú. 2022 kl. 11:59 skrifaði Greg KH <gregkh@linuxfoundation.org>:
> >
> > On Wed, Aug 17, 2022 at 11:32:48AM +0000, Andri Yngvason wrote:
> > > This fixes a race with the release-timer by adding acquire/release
> > > barrier semantics.
> >
> > What race?
> >
> 
> The race is between the release timer and processing of hid input. It
> is quite certain that these atomic checks are broken as is because
> they're lacking memory barriers and this patch does fix an actual
> problem for me.

Perhaps you should say that in this changelog then.

> 
> I must admit that I don't know exactly where the execution reordering
> occurs and I haven't checked the generated assembly code, but if you
> look at e.g. mt_expired_timeout(), you'll see that there's nothing to
> keep the compiler or CPU from hoisting the test_bit() call above the
> test_and_set_bit() call.
> 
> > >
> > > I noticed that contacts were sometimes sticking, even with the "sticky
> > > fingers" quirk enabled. This fixes that problem.
> > >
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Andri Yngvason <andri@yngvason.is>
> > > ---
> > >  drivers/hid/hid-multitouch.c | 8 ++++----
> > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c
> > > index 2e72922e36f5..91a4d3fc30e0 100644
> > > --- a/drivers/hid/hid-multitouch.c
> > > +++ b/drivers/hid/hid-multitouch.c
> > > @@ -1186,7 +1186,7 @@ static void mt_touch_report(struct hid_device *hid,
> > >       int contact_count = -1;
> > >
> > >       /* sticky fingers release in progress, abort */
> > > -     if (test_and_set_bit(MT_IO_FLAGS_RUNNING, &td->mt_io_flags))
> > > +     if (test_and_set_bit_lock(MT_IO_FLAGS_RUNNING, &td->mt_io_flags))
> >
> > So this is now a lock?
> >
> > Why not just use a real lock instead?
> >
> 
> I don't know. This stuff was introduced in
> 9609827458c37d7b2c37f2a9255631c603a5004c by Benjamin Tissoires.

Then this needs a "Fixes:" tag?

thanks,

greg k-h