Message-Id: <20251016165646.430267-1-ankur.a.arora@oracle.com>
Date: Thu, 16 Oct 2025 09:56:39 -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, 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: [PATCH v6 0/7] barrier: Add smp_cond_load_*_timeout()

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(-)

-- 
2.43.5

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