Subject: Re: [PATCH v10 00/12] barrier: Add smp_cond_load_{relaxed,acquire}_timeout()

From: Ankur Arora <[email-masked]>
Date: Thu, 02 Apr 2026 00:01:28 -0700

Ankur Arora <[email-masked]> writes:

> Hi,
>
> This series adds waited variants of the smp_cond_load() primitives:
> smp_cond_load_relaxed_timeout(), and smp_cond_load_acquire_timeout().
>
> With this version, the main remaining things are:
>
>   - Review by PeterZ of the new interface tif_need_resched_relaxed_wait()
>     (patch 11, "sched: add need-resched timed wait interface").
>
>   - Review of the BPF changes. This version simplifies the rqspinlock
>     changes by reusing the original error handling path
>     (patches 9, 10 "bpf/rqspinlock: switch check_timeout() to a clock
>     interface", "bpf/rqspinlock: Use smp_cond_load_acquire_timeout()").
>
>   - Review of WFET handling. (patch 4, "arm64: support WFET in
>     smp_cond_load_relaxed_timeout()").

Received Acks/R-bys for the two above.

Will send out an updated v11 which adds a testcase and a comment
mentioning that smp_cond_load_*_timeout on MMIO addresses might
break in interesting platform specific ways.

Thanks
Ankur

> 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(). There are, however, architectures (ex. arm64)
> that allow waiting on a cacheline instead.
>
> 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_expr, timeout)
>    smp_cond_load_acquire_timeout(ptr, cond_expr, time_expr, timeout)
>
> The parameters, time_expr, timeout determine when to bail out.
>
> Also add tif_need_resched_relaxed_wait() which wraps the pattern used
> in poll_idle() and abstracts out details of the interface and those
> of the scheduler.
>
> In addition add atomic_cond_read_*_timeout(), atomic64_cond_read_*_timeout(),
> and atomic_long wrappers to the interfaces.
>
> Finally update poll_idle() and resilient queued spinlocks to use them.
>
> Changelog:
>   v9 [9]:
>    - s/@cond/@cond_expr/ (Randy Dunlap)
>    - Clarify that SMP_TIMEOUT_POLL_COUNT is only around memory
>      addresses. (David Laight)
>    - Add the missing config ARCH_HAS_CPU_RELAX in arch/arm64/Kconfig.
>      (Catalin Marinas).
>    - Switch to arch_counter_get_cntvct_stable() (via __delay_cycles())
>      in the cmpwait path instead of using arch_timer_read_counter().
>      (Catalin Marinas)
>
>   v8 [0]:
>    - Defer evaluation of @time_expr_ns to when we hit the slowpath.
>       (comment from Alexei Starovoitov).
>
>    - Mention that cpu_poll_relax() is better than raw CPU polling
>      only where ARCH_HAS_CPU_RELAX is defined.
>      - also define ARCH_HAS_CPU_RELAX for arm64.
>       (Came out of a discussion with Will Deacon.)
>
>    - Split out WFET and WFE handling. I was doing both of these
>      in a common handler.
>      (From Will Deacon and in an earlier revision by Catalin Marinas.)
>
>    - Add mentions of atomic_cond_read_{relaxed,acquire}(),
>      atomic_cond_read_{relaxed,acquire}_timeout() in
>      Documentation/atomic_t.txt.
>
>    - Use the BIT() macro to do the checking in tif_bitset_relaxed_wait().
>
>    - Cleanup unnecessary assignments, casts etc in poll_idle().
>      (From Rafael Wysocki.)
>
>    - Fixup warnings from kernel build robot
>
>
>   v7 [1]:
>    - change the interface to separately provide the timeout. This is
>      useful for supporting WFET and similar primitives which can do
>      timed waiting (suggested by Arnd Bergmann).
>
>    - Adapting rqspinlock code to this changed interface also
>      necessitated allowing time_expr to fail.
>    - rqspinlock changes to adapt to the new smp_cond_load_acquire_timeout().
>
>    - add WFET support (suggested by Arnd Bergmann).
>    - add support for atomic-long wrappers.
>    - add a new scheduler interface tif_need_resched_relaxed_wait() which
>      encapsulates the polling logic used by poll_idle().
>      - interface suggested by (Rafael J. Wysocki).
>
>
>   v6 [2]:
>    - fixup missing timeout parameters in atomic64_cond_read_*_timeout()
>    - remove a race between setting of TIF_NEED_RESCHED and the call to
>      smp_cond_load_relaxed_timeout(). This would mean that dev->poll_time_limit
>      would be set even if we hadn't spent any time waiting.
>      (The original check compared against local_clock(), which would have been
>      fine, but I was instead using a cheaper check against _TIF_NEED_RESCHED.)
>    (Both from meta-CI bot)
>
>
>   v5 [3]:
>    - 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 [4]:
>     - 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 [5]:
>     - further interface simplifications (suggested by Catalin Marinas)
>
>   v2 [6]:
>     - 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 [7]:
>      - 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. [8]
>
> Comments appreciated!
>
> Thanks
> Ankur
>
>  [0] https://lore.kernel.org/lkml/[email-masked]/
>  [1] https://lore.kernel.org/lkml/[email-masked]/
>  [2] https://lore.kernel.org/lkml/[email-masked]/
>  [3] https://lore.kernel.org/lkml/[email-masked]/
>  [4] https://lore.kernel.org/lkml/[email-masked]/
>  [5] https://lore.kernel.org/lkml/[email-masked]/
>  [6] https://lore.kernel.org/lkml/[email-masked]/
>  [7] https://lore.kernel.org/lkml/[email-masked]/
>  [8] https://lore.kernel.org/lkml/[email-masked]/
>  [9] https://lore.kernel.org/lkml/[email-masked]/
>
> Cc: Arnd Bergmann <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> Cc: Catalin Marinas <[email-masked]>
> Cc: Peter Zijlstra <[email-masked]>
> Cc: "Rafael J. Wysocki" <[email-masked]>
> Cc: Daniel Lezcano <[email-masked]>
> Cc: Kumar Kartikeya Dwivedi <[email-masked]>
> Cc: Alexei Starovoitov <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: [email-masked]
>
> Ankur Arora (12):
>   asm-generic: barrier: Add smp_cond_load_relaxed_timeout()
>   arm64: barrier: Support smp_cond_load_relaxed_timeout()
>   arm64/delay: move some constants out to a separate header
>   arm64: support WFET in 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()
>   locking/atomic: scripts: build atomic_long_cond_read_*_timeout()
>   bpf/rqspinlock: switch check_timeout() to a clock interface
>   bpf/rqspinlock: Use smp_cond_load_acquire_timeout()
>   sched: add need-resched timed wait interface
>   cpuidle/poll_state: Wait for need-resched via
>     tif_need_resched_relaxed_wait()
>
>  Documentation/atomic_t.txt           | 14 +++--
>  arch/arm64/Kconfig                   |  3 +
>  arch/arm64/include/asm/barrier.h     | 23 +++++++
>  arch/arm64/include/asm/cmpxchg.h     | 62 +++++++++++++++----
>  arch/arm64/include/asm/delay-const.h | 27 +++++++++
>  arch/arm64/include/asm/rqspinlock.h  | 85 --------------------------
>  arch/arm64/lib/delay.c               | 15 ++---
>  drivers/cpuidle/poll_state.c         | 21 +------
>  drivers/soc/qcom/rpmh-rsc.c          |  8 +--
>  include/asm-generic/barrier.h        | 90 ++++++++++++++++++++++++++++
>  include/linux/atomic.h               | 10 ++++
>  include/linux/atomic/atomic-long.h   | 18 +++---
>  include/linux/sched/idle.h           | 29 +++++++++
>  kernel/bpf/rqspinlock.c              | 77 +++++++++++++++---------
>  scripts/atomic/gen-atomic-long.sh    | 16 +++--
>  15 files changed, 320 insertions(+), 178 deletions(-)
>  create mode 100644 arch/arm64/include/asm/delay-const.h

--
ankur