[GIT PULL] Add two-byte cmpxchg emulation function for v7.4

[Paul E. McKenney]

Hello, Linus,

When the v7.4 merge window opens, please pull this two-byte cmpxchg
emulation:

  git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git tags/cmpxchg-emu.2026.10.12b
  # HEAD: 558ccc91ddf98d58ae726ed7ccaf5838f284084f: xtensa: Emulate two-byte cmpxchg (2026-10-12 16:36:53 -0700)

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Add emulation of two-byte cmpxchg()

These commits add a cmpxchg_emu_u16() function that emulates two-byte
cmpxchg() in terms of four-byte cmpxchg() for architectures not having
native instructions for this purpose.  They also make use of this
cmpxchg_emu_u16() function in those architectures, namely sh, arc, csky,
and xtensa.  This permits use of two-byte cmpxchg() in common code.

However, it sadly does not permit easy implementation of all algorithms
needing on two-byte atomic operations because there are some systems still
supported by Linux that do not have native two-byte store instructions.
There is reason to believe that these systems will soon no longer be
supported by Linux.

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Bradley Morgan (5):
      lib: Add two-byte cmpxchg emulation function
      sh: Emulate two-byte cmpxchg
      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/sh/include/asm/cmpxchg.h     |  2 ++
 arch/xtensa/include/asm/cmpxchg.h |  1 +
 include/linux/cmpxchg-emu.h       |  4 +++-
 lib/cmpxchg-emu.c                 | 38 ++++++++++++++++++++++++++++++++++----
 6 files changed, 52 insertions(+), 5 deletions(-)