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

[Paul E. McKenney]

On Mon, Oct 05, 2026 at 07:02:05PM +0100, David Laight wrote:
> On Mon,  5 Oct 2026 12:06:57 +0000
> 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(),
> 
> That ought to be a bug...

Perhaps it is?  Here is the code:

	cmpxchg_release(&inode->i_link, NULL, pstr.name)

And inode is a struct inode, of which ->i_link is char*.  For its part,
pstr is a struct fscrypt_str, of which ->name is unsigned char *.

But this is either a 4-byte or 8-byte cmpxchg_release(), depending on
CONFIG_64BIT, which is orthogonal to this patch series, which provides
2-byte emulation.

Therefore, I see no reason to hold up these cmpxchg_emu_u16() patches.

But please let me know if I am missing something.

							Thanx, Paul

> 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(+)
> > 
>