linux-kernel - Re: [PATCH v6 0/7] barrier: Add smp_cond_load_*

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Message-ID: <875xcew8xo.fsf@oracle.com>
Date: Thu, 16 Oct 2025 19:36:51 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org,
        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, rafael@...nel.org,
        daniel.lezcano@...aro.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: Re: [PATCH v6 0/7] barrier: Add smp_cond_load_*_timeout()


An "AI" review bot flagged a couple of errors in the series: missing
param (patch 5), and a possible race in the poll_idle() (patch 7).

Let me quickly resend with those fixed.

Thanks
Ankur

Ankur Arora <ankur.a.arora@...cle.com> writes:

> This series adds waited variants of the smp_cond_load() primitives:
> smp_cond_load_relaxed_timeout(), and smp_cond_load_acquire_timeout().
>
> 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.
>
> The interfaces are:
>    smp_cond_load_relaxed_timeout(ptr, cond_expr, time_check_expr)
>    smp_cond_load_acquire_timeout(ptr, cond_expr, time_check_expr)
>
> The added parameter, time_check_expr, determines the bail out condition.
>
> Also add the ancillary interfaces atomic_cond_read_*_timeout() and,
> atomic64_cond_read_*_timeout(), both of which are wrappers around
> smp_cond_load_*_timeout().
>
> Update poll_idle() and resilient queued spinlocks to use these
> interfaces.
>
> Changelog:
>
>   v5 [1]:
>    - use cpu_poll_relax() instead of cpu_relax().
>    - instead of defining an arm64 specific
>      smp_cond_load_relaxed_timeout(), just define the appropriate
>      cpu_poll_relax().
>    - re-read the target pointer when we exit due to the time-check.
>    - s/SMP_TIMEOUT_SPIN_COUNT/SMP_TIMEOUT_POLL_COUNT/
>    (Suggested by Will Deacon)
>
>    - add atomic_cond_read_*_timeout() and atomic64_cond_read_*_timeout()
>      interfaces.
>    - rqspinlock: use atomic_cond_read_acquire_timeout().
>    - cpuidle: use smp_cond_load_relaxed_tiemout() for polling.
>    (Suggested by Catalin Marinas)
>
>    - rqspinlock: define SMP_TIMEOUT_POLL_COUNT to be 16k for non arm64
>
>   v4 [2]:
>     - naming change 's/timewait/timeout/'
>     - resilient spinlocks: get rid of res_smp_cond_load_acquire_waiting()
>       and fixup use of RES_CHECK_TIMEOUT().
>     (Both suggested by Catalin Marinas)
>
>   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 tested v4 of this series with poll_idle()/haltpoll patches. [6]
>
> Any comments appreciated!
>
> Thanks!
> Ankur
>
>  [1] https://lore.kernel.org/lkml/20250911034655.3916002-1-ankur.a.arora@oracle.com/
>  [2] https://lore.kernel.org/lkml/20250829080735.3598416-1-ankur.a.arora@oracle.com/
>  [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/2cecbf7fb23ee83a4ce027e1be3f46f97efd585c.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: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: linux-arch@...r.kernel.org
>
> Ankur Arora (7):
>   asm-generic: barrier: Add smp_cond_load_relaxed_timeout()
>   arm64: barrier: Add smp_cond_load_relaxed_timeout()
>   arm64: rqspinlock: Remove private copy of
>     smp_cond_load_acquire_timewait
>   asm-generic: barrier: Add smp_cond_load_acquire_timeout()
>   atomic: add atomic_cond_read_*_timeout()
>   rqspinlock: use smp_cond_load_acquire_timeout()
>   cpuidle/poll_state: poll via smp_cond_load_relaxed_timeout()
>
>  arch/arm64/include/asm/barrier.h    | 13 +++++
>  arch/arm64/include/asm/rqspinlock.h | 85 -----------------------------
>  drivers/cpuidle/poll_state.c        | 31 +++--------
>  include/asm-generic/barrier.h       | 63 +++++++++++++++++++++
>  include/linux/atomic.h              |  8 +++
>  kernel/bpf/rqspinlock.c             | 29 ++++------
>  6 files changed, 105 insertions(+), 124 deletions(-)


--
ankur