From: David Laight <david.laight.linux@gmail.com>
To: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, linux-pm@vger.kernel.org,
bpf@vger.kernel.org, arnd@arndb.de, catalin.marinas@arm.com,
will@kernel.org, peterz@infradead.org, mark.rutland@arm.com,
harisokn@amazon.com, cl@gentwo.org, ast@kernel.org,
rafael@kernel.org, daniel.lezcano@linaro.org, memxor@gmail.com,
zhenglifeng1@huawei.com, xueshuai@linux.alibaba.com,
rdunlap@infradead.org, joao.m.martins@oracle.com,
boris.ostrovsky@oracle.com, konrad.wilk@oracle.com
Subject: Re: [PATCH v10 00/12] barrier: Add smp_cond_load_{relaxed,acquire}_timeout()
Date: Mon, 16 Mar 2026 23:37:12 +0000 [thread overview]
Message-ID: <20260316233712.7cbfac27@pumpkin> (raw)
In-Reply-To: <874imftol4.fsf@oracle.com>
On Mon, 16 Mar 2026 15:08:07 -0700
Ankur Arora <ankur.a.arora@oracle.com> wrote:
...
> However, as David Laight pointed out in this thread
> (https://lore.kernel.org/lkml/20260214113122.70627a8b@pumpkin/)
> that this would be fine so long as the polling is on memory, but would
> need some work to handle MMIO.
I'm not sure the current code works with MMIO on arm64.
I was looking at the osq_lock() code, it uses smp_cond_load() with 'expr'
being 'VAL || need_resched()' expecting to get woken by the IPI associated
with the preemption being requested.
But the arm64 code relies on 'wfe' being woken when the memory write
'breaks' the 'ldx' for the monitored location.
That will only work for cached addresses.
For osq_lock(), while an IPI will wake it up, there is also a small timing
window where the IPI can happen before the ldx and so not actually wake up it.
This is true whenever 'expr' is non-trivial.
On arm64 I think you could use explicit sev and wfe - but that will wake all
'sleeping' cpu; and you may not want the 'thundering herd'.
David
next prev parent reply other threads:[~2026-03-16 23:37 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-16 1:36 [PATCH v10 00/12] barrier: Add smp_cond_load_{relaxed,acquire}_timeout() Ankur Arora
2026-03-16 1:36 ` [PATCH v10 01/12] asm-generic: barrier: Add smp_cond_load_relaxed_timeout() Ankur Arora
2026-03-16 1:36 ` [PATCH v10 02/12] arm64: barrier: Support smp_cond_load_relaxed_timeout() Ankur Arora
2026-03-16 1:36 ` [PATCH v10 03/12] arm64/delay: move some constants out to a separate header Ankur Arora
2026-03-16 1:36 ` [PATCH v10 04/12] arm64: support WFET in smp_cond_load_relaxed_timeout() Ankur Arora
2026-03-16 1:36 ` [PATCH v10 05/12] arm64: rqspinlock: Remove private copy of smp_cond_load_acquire_timewait() Ankur Arora
2026-03-24 1:41 ` Kumar Kartikeya Dwivedi
2026-03-25 5:58 ` Ankur Arora
2026-03-16 1:36 ` [PATCH v10 06/12] asm-generic: barrier: Add smp_cond_load_acquire_timeout() Ankur Arora
2026-03-16 1:36 ` [PATCH v10 07/12] atomic: Add atomic_cond_read_*_timeout() Ankur Arora
2026-03-16 1:36 ` [PATCH v10 08/12] locking/atomic: scripts: build atomic_long_cond_read_*_timeout() Ankur Arora
2026-03-16 1:36 ` [PATCH v10 09/12] bpf/rqspinlock: switch check_timeout() to a clock interface Ankur Arora
2026-03-24 1:43 ` Kumar Kartikeya Dwivedi
2026-03-25 5:57 ` Ankur Arora
2026-03-16 1:36 ` [PATCH v10 10/12] bpf/rqspinlock: Use smp_cond_load_acquire_timeout() Ankur Arora
2026-03-24 1:46 ` Kumar Kartikeya Dwivedi
2026-03-16 1:36 ` [PATCH v10 11/12] sched: add need-resched timed wait interface Ankur Arora
2026-03-16 1:36 ` [PATCH v10 12/12] cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed_wait() Ankur Arora
2026-03-16 1:49 ` [PATCH v10 00/12] barrier: Add smp_cond_load_{relaxed,acquire}_timeout() Andrew Morton
2026-03-16 22:08 ` Ankur Arora
2026-03-16 23:37 ` David Laight [this message]
2026-03-17 6:53 ` Ankur Arora
2026-03-17 9:17 ` David Laight
2026-03-25 13:53 ` Catalin Marinas
2026-03-25 15:42 ` David Laight
2026-03-25 16:32 ` Catalin Marinas
2026-03-25 20:23 ` David Laight
2026-03-26 15:39 ` Catalin Marinas
2026-03-25 15:55 ` Catalin Marinas
2026-03-25 19:36 ` David Laight
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