linux-kernel - [PATCH v4 0/5] barrier: Add smp_cond_load_*

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Message-Id: <20250829080735.3598416-1-ankur.a.arora@oracle.com>
Date: Fri, 29 Aug 2025 01:07:30 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org
Cc: arnd@...db.de, catalin.marinas@....com, will@...nel.org,
        peterz@...radead.org, akpm@...ux-foundation.org, mark.rutland@....com,
        harisokn@...zon.com, cl@...two.org, ast@...nel.org, memxor@...il.com,
        zhenglifeng1@...wei.com, xueshuai@...ux.alibaba.com,
        joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: [PATCH v4 0/5] barrier: Add smp_cond_load_*_timewait()

Hi,

This series adds waited variants of the smp_cond_load() primitives:
smp_cond_load_relaxed_timewait(), and smp_cond_load_acquire_timewait().

Why?: as the name suggests, the new interfaces are meant for contexts
where you want to wait on a condition variable for a finite duration.
This is easy enough to do with a loop around cpu_relax(). However,
some architectures (ex. arm64) also allow waiting on a cacheline. So,
these interfaces handle a mixture of spin/wait with a smp_cond_load()
thrown in.

There are two known users for these interfaces:

 - poll_idle() [1]
 - resilient queued spinlocks [2]

The interfaces are:
   smp_cond_load_relaxed_timewait(ptr, cond_expr, time_check_expr)
   smp_cond_load_acquire_spinwait(ptr, cond_expr, time_check_expr)

The added parameter, time_check_expr, determines the bail out condition.

Changelog:
  v3 [3]:
    - further interface simplifications (suggested by Catalin Marinas)

  v2 [4]:
    - simplified the interface (suggested by Catalin Marinas)
       - get rid of wait_policy, and a multitude of constants
       - adds a slack parameter
      This helped remove a fair amount of duplicated code duplication and in hindsight
      unnecessary constants.

  v1 [5]:
     - add wait_policy (coarse and fine)
     - derive spin-count etc at runtime instead of using arbitrary
       constants.

Haris Okanovic had tested an earlier version of this series with 
poll_idle()/haltpoll patches. [6]

Any comments appreciated!

Thanks!
Ankur

[1] https://lore.kernel.org/lkml/20241107190818.522639-3-ankur.a.arora@oracle.com/
[2] Uses the smp_cond_load_acquire_timewait() from v1
    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/include/asm/rqspinlock.h
[3] https://lore.kernel.org/lkml/20250627044805.945491-1-ankur.a.arora@oracle.com/
[4] https://lore.kernel.org/lkml/20250502085223.1316925-1-ankur.a.arora@oracle.com/
[5] https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com/
[6] https://lore.kernel.org/lkml/f2f5d09e79539754ced085ed89865787fa668695.camel@amazon.com

Cc: Arnd Bergmann <arnd@...db.de>
Cc: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: linux-arch@...r.kernel.org

Ankur Arora (5):
  asm-generic: barrier: Add smp_cond_load_relaxed_timewait()
  arm64: barrier: Add smp_cond_load_relaxed_timewait()
  arm64: rqspinlock: Remove private copy of
    smp_cond_load_acquire_timewait
  asm-generic: barrier: Add smp_cond_load_acquire_timewait()
  rqspinlock: use smp_cond_load_acquire_timewait()

 arch/arm64/include/asm/barrier.h    | 22 ++++++++
 arch/arm64/include/asm/rqspinlock.h | 84 +----------------------------
 include/asm-generic/barrier.h       | 57 ++++++++++++++++++++
 include/asm-generic/rqspinlock.h    |  4 ++
 kernel/bpf/rqspinlock.c             | 25 ++++-----
 5 files changed, 93 insertions(+), 99 deletions(-)

-- 
2.31.1