linux-kernel - Re: [PATCH v4 5/5] rqspinlock: use smp_cond_load_acquire

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Message-ID: <aLWDcJiZWD7g8-4S@arm.com>
Date: Mon, 1 Sep 2025 12:28:48 +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 v4 5/5] rqspinlock: use smp_cond_load_acquire_timewait()

On Fri, Aug 29, 2025 at 01:07:35AM -0700, Ankur Arora wrote:
> diff --git a/arch/arm64/include/asm/rqspinlock.h b/arch/arm64/include/asm/rqspinlock.h
> index a385603436e9..ce8feadeb9a9 100644
> --- a/arch/arm64/include/asm/rqspinlock.h
> +++ b/arch/arm64/include/asm/rqspinlock.h
> @@ -3,6 +3,9 @@
>  #define _ASM_RQSPINLOCK_H
>  
>  #include <asm/barrier.h>
> +
> +#define res_smp_cond_load_acquire_waiting() arch_timer_evtstrm_available()

More on this below, I don't think we should define it.

> diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c
> index 5ab354d55d82..8de1395422e8 100644
> --- a/kernel/bpf/rqspinlock.c
> +++ b/kernel/bpf/rqspinlock.c
> @@ -82,6 +82,7 @@ struct rqspinlock_timeout {
>  	u64 duration;
>  	u64 cur;
>  	u16 spin;
> +	u8  wait;
>  };
>  
>  #define RES_TIMEOUT_VAL	2
> @@ -241,26 +242,20 @@ static noinline int check_timeout(rqspinlock_t *lock, u32 mask,
>  }
>  
>  /*
> - * Do not amortize with spins when res_smp_cond_load_acquire is defined,
> - * as the macro does internal amortization for us.
> + * Only amortize with spins when we don't have a waiting implementation.
>   */
> -#ifndef res_smp_cond_load_acquire
>  #define RES_CHECK_TIMEOUT(ts, ret, mask)                              \
>  	({                                                            \
> -		if (!(ts).spin++)                                     \
> +		if ((ts).wait || !(ts).spin++)		      \
>  			(ret) = check_timeout((lock), (mask), &(ts)); \
>  		(ret);                                                \
>  	})
> -#else
> -#define RES_CHECK_TIMEOUT(ts, ret, mask)			      \
> -	({ (ret) = check_timeout((lock), (mask), &(ts)); })
> -#endif

IIUC, RES_CHECK_TIMEOUT in the current res_smp_cond_load_acquire() usage
doesn't amortise the spins, as the comment suggests, but rather the
calls to check_timeout(). This is fine, it matches the behaviour of
smp_cond_load_relaxed_timewait() you introduced in the first patch. The
only difference is the number of spins - 200 (matching poll_idle) vs 64K
above. Does 200 work for the above?

>  /*
>   * Initialize the 'spin' member.
>   * Set spin member to 0 to trigger AA/ABBA checks immediately.
>   */
> -#define RES_INIT_TIMEOUT(ts) ({ (ts).spin = 0; })
> +#define RES_INIT_TIMEOUT(ts) ({ (ts).spin = 0; (ts).wait = res_smp_cond_load_acquire_waiting(); })

First of all, I don't really like the smp_cond_load_acquire_waiting(),
that's an implementation detail of smp_cond_load_*_timewait() that
shouldn't leak outside. But more importantly, RES_CHECK_TIMEOUT() is
also used outside the smp_cond_load_acquire_timewait() condition. The
(ts).wait check only makes sense when used together with the WFE
waiting.

I would leave RES_CHECK_TIMEOUT() as is for the stand-alone cases and
just use check_timeout() in the smp_cond_load_acquire_timewait()
scenarios. I would also drop the res_smp_cond_load_acquire() macro since
you now defined smp_cond_load_acquire_timewait() generically and can be
used directly.

-- 
Catalin