[RESEND PATCH v7 0/7] barrier: Add smp_cond_load_*_timeout()

Ankur Arora
2025-10-28T05:32:22Z

[ Added linux-pm to Cc for the poll_idle() changes. ]

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:

v6 [1]:
- 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 [2]:
- 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 [3]:
- 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 [4]:
- further interface simplifications (suggested by Catalin Marinas)

v2 [5]:
- 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 [6]:
- 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. [7]

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/20250911034655.3916002-1-ankur.a.arora@oracle.com/
[3] https://lore.kernel.org/lkml/20250829080735.3598416-1-ankur.a.arora@oracle.com/
[4] https://lore.kernel.org/lkml/20250627044805.945491-1-ankur.a.arora@oracle.com/
[5] https://lore.kernel.org/lkml/20250502085223.1316925-1-ankur.a.arora@oracle.com/
[6] https://lore.kernel.org/lkml/20250203214911.898276-1-ankur.a.arora@oracle.com/
[7] https://lore.kernel.org/lkml/2cecbf7fb23ee83a4ce027e1be3f46f97efd585c.camel@amazon.com/

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: linux-arch@vger.kernel.org

Ankur Arora (7):
asm-generic: barrier: Add smp_cond_load_relaxed_timeout()
arm64: barrier: Support 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 | 29 +++-------
include/asm-generic/barrier.h | 63 +++++++++++++++++++++
include/linux/atomic.h | 10 ++++
kernel/bpf/rqspinlock.c | 31 +++++------
6 files changed, 108 insertions(+), 123 deletions(-)

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2.43.5