[PATCH 3/5] ARC: Emulate two-byte cmpxchg

[Paul E. McKenney]

On Wed, Oct 07, 2026 at 10:48:39PM +0200, Vineet Gupta wrote:
> On 10/7/26 22:36, Paul E. McKenney wrote:
> > From: Bradley Morgan <brads at mainlining.org>
> > 
> > ARC has no two-byte atomic compare and swap, so the
> > arch_cmpxchg_relaxed() macro switch lets case 2 fall through to
> > BUILD_BUG() via default, and a two-byte cmpxchg() fails the build.
> > 
> > Route case 2 through the new cmpxchg_emu_u16(), which narrows the
> > old and new values itself, so the dispatch keeps the (uintptr_t)
> > casts the case 1 path already uses.
> > 
> > The sizeof(*(_p_)) fix that v4 folded in here landed separately
> > through Vineet's f050c3e61d2a, so this patch is now only the
> > case 2 wiring.
> > 
> > Signed-off-by: Bradley Morgan <brads at mainlining.org>
> > Signed-off-by: Paul E. McKenney <paulmck at kernel.org>
> 
> Acked-by: Vineet Gupta <vgupta at kernel.org>

Thank you!  I will apply this on my next rebase.  Unless something breaks,
I will include this in my pull request for the upcoming merge window.

							Thanx, Paul

> Thx,
> -Vineet
> 
> > ---
> >   arch/arc/include/asm/cmpxchg.h | 3 +++
> >   1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
> > index 76f43db0890fc..7157718f147c1 100644
> > --- a/arch/arc/include/asm/cmpxchg.h
> > +++ b/arch/arc/include/asm/cmpxchg.h
> > @@ -50,6 +50,9 @@
> >   	case 1:								\
> >   		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *__force)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
> >   		break;							\
> > +	case 2:								\
> > +		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *__force)_p_, (unsigned long)_o_, (unsigned long)_n_);	\
> > +		break;							\
> >   	case 4:								\
> >   		_prev_ = __cmpxchg(_p_, _o_, _n_);			\
> >   		break;							\
>