linux-kernel - Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation

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Message-ID: <YFsd7A6xzrWXi1l5@hirez.programming.kicks-ass.net>
Date:   Wed, 24 Mar 2021 12:09:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     guoren@...nel.org
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Guo Ren <guoren@...ux.alibaba.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Anup Patel <anup@...infault.org>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] riscv: locks: introduce ticket-based spinlock
 implementation

On Wed, Mar 24, 2021 at 10:14:52AM +0000, guoren@...nel.org wrote:
> +static inline void arch_spin_lock(arch_spinlock_t *lock)
> +{
> +	arch_spinlock_t lockval;
> +	u32 tmp;
> +
> +	asm volatile (
> +		"1:	lr.w	%0, %2		\n"
> +		"	mv	%1, %0		\n"
> +		"	addw	%0, %0, %3	\n"
> +		"	sc.w	%0, %0, %2	\n"
> +		"	bnez	%0, 1b		\n"
> +		: "=&r" (tmp), "=&r" (lockval), "+A" (lock->lock)
> +		: "r" (1 << TICKET_NEXT)
> +		: "memory");
>  
> +	while (lockval.tickets.next != lockval.tickets.owner) {
> +		/*
> +		 * FIXME - we need wfi/wfe here to prevent:
> +		 *  - cache line bouncing
> +		 *  - saving cpu pipeline in multi-harts-per-core
> +		 *    processor
> +		 */
> +		lockval.tickets.owner = READ_ONCE(lock->tickets.owner);
> +	}
>  
> +	__atomic_acquire_fence();
>  }

> +static inline void arch_spin_unlock(arch_spinlock_t *lock)
>  {
> +	smp_store_release(&lock->tickets.owner, lock->tickets.owner + 1);
> +	/* FIXME - we need ipi/sev here to notify above */
>  }

Urgh, are you saying your WFE requires an explicit SEV like on ARM ? The
ARM64 model is far superious IMO, and then you can use
smp_cond_load_acquire() in arch_spin_lock() and call it a day.