Re: [PATCH v5 0/3] Add two-byte cmpxchg emulation and wire it into the architectures

[David Laight <david.laight.linux@gmail.com>]

On Mon,  5 Oct 2026 12:06:57 +0000
Bradley Morgan <brads@mainlining.org> wrote:

> This is v5 of the two-byte cmpxchg emulation series, reduced to the
> three architectures still missing after Paul McKenney queued the lib
> and sh patches, ARC, csky and xtensa.
> 
> The v4 attempt at these folded in a type checking idiom,
> (unsigned long)(0 ? *ptr : old), meant to make cmpxchg(&p, 4, 5) fail
> to compile. The kernel test robot and Vineet Gupta showed that idiom
> breaks real callers, fs/crypto/hooks.c passes a char * and an
> unsigned char * to cmpxchg_release(),

That ought to be a bug...

David

> and the conditional expression
> then has incompatible pointer types, which is a hard error on gcc 14
> and newer. So this version only adds the case 2 dispatch, the
> declarations the architectures already have are kept as is.
> 
> The ARC sizeof bug that v4 fixed along the way is in mainline
> separately through Vineet's f050c3e61d2a ("ARC: arch_cmpxchg_relaxed
> to use size of pointed type not pointer"), so the ARC patch here is
> only the case 2 wiring on top of that.
> 
> On the why, RCU previously used single-byte cmpxchg(), which is what
> motivated cmpxchg_emu_u8() in the first place, and Paul has now
> queued cmpxchg_emu_u16(). Unused new code is frowned upon, so wiring
> it into the architectures that need it is the missing half, and
> there are existing workarounds for the missing two-byte cmpxchg() in
> the tree, _Q_PENDING_BITS for one, that can make use of it.
> 
> Per Paul's suggestion each patch is standalone and can go in
> independently, they only depend on the lib patch already queued.
> Each one was build tested with the real cross toolchain, ARC with
> arc-linux-gnu-gcc and csky and xtensa with the gcc 16.2 crosstool
> builds Vineet pointed at, W=1, with the macro instantiated on
> u8, u16, u32 and pointer types, confirming the new dispatch is
> reached and no new warnings appear. The pointer instantiation
> covers the fs/crypto/hooks.c case that broke v4.
> 
> Bradley Morgan (3):
>   ARC: Emulate two-byte cmpxchg
>   csky: Emulate two-byte cmpxchg
>   xtensa: Emulate two-byte cmpxchg
> 
>  arch/arc/include/asm/cmpxchg.h    | 3 +++
>  arch/csky/include/asm/cmpxchg.h   | 9 +++++++++
>  arch/xtensa/include/asm/cmpxchg.h | 1 +
>  3 files changed, 13 insertions(+)
>