linux-kernel - [PATCH RFC cmpxchg 0/8] Provide emulation for one- and two-byte cmpxchg()

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Date: Mon, 1 Apr 2024 14:39:21 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: elver@...gle.com, akpm@...ux-foundation.org, tglx@...utronix.de,
	peterz@...radead.org, dianders@...omium.org, pmladek@...e.com,
	torvalds@...ux-foundation.org
Subject: [PATCH RFC cmpxchg 0/8] Provide emulation for one- and two-byte
 cmpxchg()

Hello!

This series provides emulation functions for one-byte and two-byte
cmpxchg, and uses them for those architectures not supporting these
in hardware.  The emulation is in terms of the fully ordered four-byte
cmpxchg() that is supplied by all of these architectures.  These were
tested by making x86 forget that it can do these natively:

88a9b3f7a924 ("EXP arch/x86: Test one-byte cmpxchg emulation")

This commit is local to -rcu and is of course not intended for mainline.

If accepted, RCU Tasks will use this capability in place of the current
rcu_trc_cmpxchg_need_qs() open-coding of this emulation.

1.	Add one-byte and two-byte cmpxchg() emulation functions.

2.	sparc: Emulate one-byte and two-byte cmpxchg.

3.	ARC: Emulate one-byte and two-byte cmpxchg.

4.	csky: Emulate one-byte and two-byte cmpxchg.

5.	sh: Emulate one-byte and two-byte cmpxchg.

6.	xtensa: Emulate one-byte and two-byte cmpxchg.

7.	parisc: Emulate two-byte cmpxchg.

8.	riscv: Emulate one-byte and two-byte cmpxchg.

						Thanx, Paul

------------------------------------------------------------------------

 arch/Kconfig                        |    3 +
 arch/arc/Kconfig                    |    1 
 arch/arc/include/asm/cmpxchg.h      |   38 ++++++++++++++----
 arch/csky/Kconfig                   |    1 
 arch/csky/include/asm/cmpxchg.h     |   18 ++++++++
 arch/parisc/Kconfig                 |    1 
 arch/parisc/include/asm/cmpxchg.h   |    1 
 arch/riscv/Kconfig                  |    1 
 arch/riscv/include/asm/cmpxchg.h    |   25 ++++++++++++
 arch/sh/Kconfig                     |    1 
 arch/sh/include/asm/cmpxchg.h       |    4 +
 arch/sparc/Kconfig                  |    1 
 arch/sparc/include/asm/cmpxchg_32.h |    6 ++
 arch/xtensa/Kconfig                 |    1 
 arch/xtensa/include/asm/cmpxchg.h   |    3 +
 include/linux/cmpxchg-emu.h         |   16 +++++++
 lib/Makefile                        |    1 
 lib/cmpxchg-emu.c                   |   74 ++++++++++++++++++++++++++++++++++++
 18 files changed, 187 insertions(+), 9 deletions(-)