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

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Message-ID: <7b3646e0-667c-48e2-8f09-e493c43c30cb@paulmck-laptop>
Date: Mon, 8 Apr 2024 10:47:46 -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, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH RFC cmpxchg 0/8] Provide emulation for one- and two-byte
 cmpxchg()

Hello!

This series provides emulation functions for one-byte cmpxchg, and uses it
for those architectures not supporting this in hardware.  The emulation
is in terms of the fully ordered four-byte cmpxchg() that is supplied
by all of these architectures.  This was tested by making x86 forget
that it can do one-byte cmpxchg() 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.	sparc32: make __cmpxchg_u32() return u32, courtesy of Al Viro.

2.	sparc32: make the first argument of __cmpxchg_u64() volatile u64 *,
	courtesy of Al Viro.

3.	sparc32: unify __cmpxchg_u{32,64}, courtesy of Al Viro.

4.	sparc32: add __cmpxchg_u{8,16}() and teach __cmpxchg() to handle those
	sizes, courtesy of Al Viro.

5.	parisc: __cmpxchg_u32(): lift conversion into the callers, courtesy of
	Al Viro.

6.	parisc: unify implementations of __cmpxchg_u{8,32,64}, courtesy of
	Al Viro.

7.	parisc: add missing export of __cmpxchg_u8(), courtesy of Al Viro.

8.	parisc: add u16 support to cmpxchg(), courtesy of Al Viro.

9.	lib: Add one-byte emulation function.

10.	ARC: Emulate one-byte cmpxchg.

11.	csky: Emulate one-byte cmpxchg.

12.	sh: Emulate one-byte cmpxchg.

13.	xtensa: Emulate one-byte cmpxchg.

14.	riscv: Emulate one-byte cmpxchg.

Changes since v1:

o	Add native support for sparc32 and parisc, courtesy of Al Viro.

o	Remove two-byte emulation due to architectures that still do not
	support two-byte load and store instructions, per Arnd Bergmann
	feedback.  (Yes, there are a few systems out there that do not
	even support one-byte load instructions, but these are slated
	for removal anyway.)

o	Fix numerous casting bugs spotted by kernel test robot.

o	Fix SPDX header.  "//" for .c files and "/*" for .h files.
	I am sure that there is a good reason for this.  ;-)

						Thanx, Paul

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

 arch/parisc/include/asm/cmpxchg.h     |   19 +++++-------
 arch/parisc/kernel/parisc_ksyms.c     |    1 
 arch/parisc/lib/bitops.c              |   52 +++++++++++-----------------------
 arch/sparc/include/asm/cmpxchg_32.h   |   18 +++++------
 arch/sparc/lib/atomic32.c             |   47 +++++++++++++-----------------
 b/arch/Kconfig                        |    3 +
 b/arch/arc/Kconfig                    |    1 
 b/arch/arc/include/asm/cmpxchg.h      |   32 +++++++++++++++-----
 b/arch/csky/Kconfig                   |    1 
 b/arch/csky/include/asm/cmpxchg.h     |   10 ++++++
 b/arch/parisc/include/asm/cmpxchg.h   |    3 -
 b/arch/parisc/kernel/parisc_ksyms.c   |    1 
 b/arch/parisc/lib/bitops.c            |    6 +--
 b/arch/riscv/Kconfig                  |    1 
 b/arch/riscv/include/asm/cmpxchg.h    |   13 ++++++++
 b/arch/sh/Kconfig                     |    1 
 b/arch/sh/include/asm/cmpxchg.h       |    2 +
 b/arch/sparc/include/asm/cmpxchg_32.h |    4 +-
 b/arch/sparc/lib/atomic32.c           |    4 +-
 b/arch/xtensa/Kconfig                 |    1 
 b/arch/xtensa/include/asm/cmpxchg.h   |    2 +
 b/include/linux/cmpxchg-emu.h         |   15 +++++++++
 b/lib/Makefile                        |    1 
 b/lib/cmpxchg-emu.c                   |   45 +++++++++++++++++++++++++++++
 24 files changed, 184 insertions(+), 99 deletions(-)