[PATCH 00/19] Introduce __xchg, non-atomic xchg

Andrzej Hajda
2022-12-22T11:47:30Z

Hi all,

I hope there will be place for such tiny helper in kernel.
Quick cocci analyze shows there is probably few thousands places
where it could be useful.
I am not sure who is good person to review/ack such patches,
so I've used my intuition to construct to/cc lists, sorry for mistakes.
This is the 2nd approach of the same idea, with comments addressed[0].

The helper is tiny and there are advices we can leave without it, so
I want to present few arguments why it would be good to have it:

1. Code readability/simplification/number of lines:

Real example from drivers/net/ethernet/mellanox/mlx5/core/esw/qos.c:
- previous_min_rate = evport->qos.min_rate;
- evport->qos.min_rate = min_rate;
+ previous_min_rate = __xchg(evport->qos.min_rate, min_rate);

For sure the code is more compact, and IMHO more readable.

2. Presence of similar helpers in other somehow related languages/libs:

a) Rust[1]: 'replace' from std::mem module, there is also 'take'
helper (__xchg(&x, 0)), which is the same as private helper in
i915 - fetch_and_zero, see latest patch.
b) C++ [2]: 'exchange' from utility header.

If the idea is OK there are still 2 qestions to answer:

1. Name of the helper, __xchg follows kernel conventions,
but for me Rust names are also OK.
2. Where to put the helper:
a) as in this patchset include/linux/non-atomic/xchg.h,
proposed by Andy Shevchenko,
b) include/linux/utils.h ? any better name? Some kind
of container for simple helpers.

Structure of the patchset:
17 patches releasing __xchg name from arch files
1 patch adding __xchg
1 patch adding users of __xchg

Arnd thanks for convienient set of cross compilers, it was very helpful.

So many words for so small helper :)

[0]: https://lore.kernel.org/lkml/Y5OFSvaYbv4XCxhE@smile.fi.intel.com/T/
[1]: https://doc.rust-lang.org/std/mem/index.html
[2]: https://en.cppreference.com/w/cpp/header/utility

Regards
Andrzej

Andrzej Hajda (19):
arch/alpha: rename internal name __xchg to __arch_xchg
arch/arc: rename internal name __xchg to __arch_xchg
arch/arm: rename internal name __xchg to __arch_xchg
arch/arm64: rename internal name __xchg to __arch_xchg
arch/hexagon: rename internal name __xchg to __arch_xchg
arch/ia64: rename internal name __xchg to __arch_xchg
arch/loongarch: rename internal name __xchg to __arch_xchg
arch/m68k: rename internal name __xchg to __arch_xchg
arch/mips: rename internal name __xchg to __arch_xchg
arch/openrisc: rename internal name __xchg to __arch_xchg
arch/parisc: rename internal name __xchg to __arch_xchg
arch/powerpc: correct logged function names in xchg helpers
arch/riscv: rename internal name __xchg to __arch_xchg
arch/s390: rename internal name __xchg to __arch_xchg
arch/sh: rename internal name __xchg to __arch_xchg
arch/sparc: rename internal name __xchg to __arch_xchg
arch/xtensa: rename internal name __xchg to __arch_xchg
linux/include: add non-atomic version of xchg
drm/i915/gt: use __xchg instead of internal helper

arch/alpha/include/asm/cmpxchg.h | 6 +++---
arch/arc/include/asm/cmpxchg.h | 4 ++--
arch/arm/include/asm/cmpxchg.h | 4 ++--
arch/arm64/include/asm/cmpxchg.h | 4 ++--
arch/hexagon/include/asm/cmpxchg.h | 6 +++---
arch/ia64/include/asm/cmpxchg.h | 2 +-
arch/ia64/include/uapi/asm/cmpxchg.h | 4 ++--
arch/loongarch/include/asm/cmpxchg.h | 4 ++--
arch/m68k/include/asm/cmpxchg.h | 6 +++---
arch/mips/include/asm/cmpxchg.h | 4 ++--
arch/openrisc/include/asm/cmpxchg.h | 4 ++--
arch/parisc/include/asm/cmpxchg.h | 4 ++--
arch/powerpc/include/asm/cmpxchg.h | 4 ++--
arch/riscv/include/asm/atomic.h | 2 +-
arch/riscv/include/asm/cmpxchg.h | 4 ++--
arch/s390/include/asm/cmpxchg.h | 4 ++--
arch/sh/include/asm/cmpxchg.h | 4 ++--
arch/sparc/include/asm/cmpxchg_32.h | 4 ++--
arch/sparc/include/asm/cmpxchg_64.h | 4 ++--
arch/xtensa/include/asm/cmpxchg.h | 4 ++--
drivers/gpu/drm/i915/gt/intel_engine_cs.c | 2 +-
.../gpu/drm/i915/gt/intel_engine_heartbeat.c | 4 ++--
.../drm/i915/gt/intel_execlists_submission.c | 4 ++--
drivers/gpu/drm/i915/gt/intel_ggtt.c | 4 ++--
drivers/gpu/drm/i915/gt/intel_gsc.c | 2 +-
drivers/gpu/drm/i915/gt/intel_gt.c | 4 ++--
drivers/gpu/drm/i915/gt/intel_gt_pm.c | 2 +-
drivers/gpu/drm/i915/gt/intel_lrc.c | 6 +++---
drivers/gpu/drm/i915/gt/intel_migrate.c | 2 +-
drivers/gpu/drm/i915/gt/intel_rc6.c | 2 +-
drivers/gpu/drm/i915/gt/intel_rps.c | 2 +-
drivers/gpu/drm/i915/gt/selftest_context.c | 2 +-
.../drm/i915/gt/selftest_ring_submission.c | 2 +-
drivers/gpu/drm/i915/gt/selftest_timeline.c | 2 +-
drivers/gpu/drm/i915/gt/uc/intel_gsc_uc.c | 2 +-
drivers/gpu/drm/i915/gt/uc/intel_uc.c | 2 +-
drivers/gpu/drm/i915/gt/uc/intel_uc_fw.c | 2 +-
drivers/gpu/drm/i915/i915_utils.h | 1 +
include/linux/non-atomic/xchg.h | 19 +++++++++++++++++++
39 files changed, 84 insertions(+), 64 deletions(-)
create mode 100644 include/linux/non-atomic/xchg.h

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2.34.1