linux-kernel - Re: [PATCH v5 0/5] barrier: Add smp_cond_load_*

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Message-ID: <87tt18k5y7.fsf@oracle.com>
Date: Thu, 11 Sep 2025 14:57:52 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        bpf@...r.kernel.org, arnd@...db.de, will@...nel.org,
        peterz@...radead.org, akpm@...ux-foundation.org, mark.rutland@....com,
        harisokn@...zon.com, cl@...two.org, ast@...nel.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 v5 0/5] barrier: Add smp_cond_load_*_timeout()


Catalin Marinas <catalin.marinas@....com> writes:

> On Wed, Sep 10, 2025 at 08:46:50PM -0700, Ankur Arora wrote:
>> 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() and a periodic
>> timeout check (pretty much what we do in poll_idle(). However, some
>> architectures (ex. arm64) also allow waiting on a cacheline. So,
>>
>>   smp_cond_load_relaxed_timeout(ptr, cond_expr, time_check_expr)
>>   smp_cond_load_acquire_timeout(ptr, cond_expr, time_check_expr)
>>
>> do a mixture of spin/wait with a smp_cond_load() thrown in.
>>
>> The added parameter, time_check_expr, determines the bail out condition.
>>
>> There are two current users for these interfaces. poll_idle() with
>> the change:
>>
>>   poll_idle() {
>>       ...
>>       time_end = local_clock_noinstr() + cpuidle_poll_time(drv, dev);
>>
>>       raw_local_irq_enable();
>>       if (!current_set_polling_and_test())
>>       	 flags = smp_cond_load_relaxed_timeout(&current_thread_info()->flags,
>>       					(VAL & _TIF_NEED_RESCHED),
>>       					((local_clock_noinstr() >= time_end)));
>>       dev->poll_time_limit = !(flags & _TIF_NEED_RESCHED);
>>       raw_local_irq_disable();
>>       ...
>>   }
>
> You should have added this as a patch in the series than include the
> implementation in the cover letter.

This was probably an overkill but I wanted to not add another subsystem
to this series.
Will take care of the cpuidle changes in the arm64 polling in idle series.

--
ankur