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

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Message-ID: <aMLd2QOFrZnaKcWf@arm.com>
Date: Thu, 11 Sep 2025 15:34:01 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: 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()

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.

-- 
Catalin