linux-kernel - Re: [RESEND PATCH v7 2/7] arm64: barrier: Support smp_cond_load_relaxed

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Message-ID: <aQoF1-uKTgJo89W8@arm.com>
Date: Tue, 4 Nov 2025 13:55:35 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	Linux-Arch <linux-arch@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org, linux-pm@...r.kernel.org,
	bpf@...r.kernel.org, Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mark Rutland <mark.rutland@....com>,
	Haris Okanovic <harisokn@...zon.com>,
	"Christoph Lameter (Ampere)" <cl@...two.org>,
	Alexei Starovoitov <ast@...nel.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Kumar Kartikeya Dwivedi <memxor@...il.com>, zhenglifeng1@...wei.com,
	xueshuai@...ux.alibaba.com,
	Joao Martins <joao.m.martins@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [RESEND PATCH v7 2/7] arm64: barrier: Support
 smp_cond_load_relaxed_timeout()

On Mon, Nov 03, 2025 at 01:00:33PM -0800, Ankur Arora wrote:
>     /**
>     * smp_cond_load_relaxed_timeout() - (Spin) wait for cond with no ordering
>     * guarantees until a timeout expires.
>     * @ptr: pointer to the variable to wait on
>     * @cond: boolean expression to wait for
>     * @time_expr: time expression in caller's preferred clock
>     * @time_end: end time in nanosecond (compared against time_expr;
>     * might also be used for setting up a future event.)
>     *
>     * Equivalent to using READ_ONCE() on the condition variable.
>     *
>     * Note that the expiration of the timeout might have an architecture specific
>     * delay.
>     */
>     #ifndef smp_cond_load_relaxed_timeout
>     #define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr, time_end_ns)	\
>     ({									\
>             typeof(ptr) __PTR = (ptr);					\
>             __unqual_scalar_typeof(*ptr) VAL;				\
>             u32 __n = 0, __spin = SMP_TIMEOUT_POLL_COUNT;		\
>             u64 __time_end_ns = (time_end_ns);				\
>                                                                         \
>             for (;;) {							\
>                     VAL = READ_ONCE(*__PTR);				\
>                     if (cond_expr)					\
>                             break;					\
>                     cpu_poll_relax(__PTR, VAL, __time_end_ns);		\

With time_end_ns being passed to cpu_poll_relax(), we assume that this
is always the absolute time. Do we still need time_expr in this case?
It works for WFET as long as we can map this time_end_ns onto the
hardware CNTVCT.

Alternatively, we could pass something like remaining_ns, though not
sure how smp_cond_load_relaxed_timeout() can decide to spin before
checking time_expr again (we probably went over this in the past two
years ;)).

>                     if (++__n < __spin)				\
>                             continue;					\
>                     if ((time_expr) >= __time_end_ns) {		\
>                             VAL = READ_ONCE(*__PTR);			\
>                             break;					\
>                     }							\
>                     __n = 0;						\
>             }								\
>             (typeof(*ptr))VAL;						\
>     })
>     #endif

-- 
Catalin