Re: [PATCH stable-4.9.y] locking/qspinlock: Pull in asm/byteorder.h to ensure correct endianness

[David Airlie]

On Fri, Jan 25, 2019 at 5:04 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Jan 24, 2019 at 06:54:15PM +0000, Will Deacon wrote:
> > From: Dave Airlie <airlied@xxxxxxxxxx>
> >
> > This commit is not required upstream, but is required for the 4.9.y
> > stable series.
> >
> > Upstream commit 101110f6271c ("Kbuild: always define endianess in
> > kconfig.h") ensures that either __LITTLE_ENDIAN or __BIG_ENDIAN is
> > defined to reflect the endianness of the target CPU architecture
> > regardless of whether or not <asm/byteorder.h> has been #included. The
> > upstream definition of 'struct qspinlock' relies on this property.
> >
> > Unfortunately, the 4.9.y stable series does not provide this guarantee,
> > so the 'spin_unlock()' routine can erroneously treat the underlying
> > lockword as big-endian on little-endian architectures using native
> > qspinlock (i.e. x86_64 without PV) if the caller has not included
> > <asm/byteorder.h>. This can lead to hangs such as the one in
> > 'i915_gem_request()' reported via bugzilla:
> >
> >   https://bugzilla.kernel.org/show_bug.cgi?id=202063
> >
> > Fix the issue by ensuring that <asm/byteorder.h> is #included in
> > <asm/qspinlock_types.h>, where 'struct qspinlock' is defined.
>
> That is crazy...

You've no idea, we've spent a couple of days at LCA figuring it all out :-)

Dave.