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

[Bradley Morgan]

On 5 October 2026 13:06:57 BST, Bradley Morgan <brads@xxxxxxxxxxxxxx>
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(), 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.

Sashiko took a hitch at it, all the "bugs" are bogus, most are just cuz
"u16 is undefined oh no error", but they didn't see Paul's tree, where it
is defined.

whatever

>
>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(+)
>
>

--- Thanks!
"I'm not a very positive person" - Linus torvalds