linux-kernel - Re: [PATCH] locking/atomic: atomic: Use arch_atomic

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Message-ID: <Y9Oy9ZAj/DQ7O+6e@hirez.programming.kicks-ass.net>
Date:   Fri, 27 Jan 2023 12:18:13 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jules Maselbas <jmaselbas@...ray.eu>
Cc:     Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/atomic: atomic: Use arch_atomic_{read,set} in
 generic atomic ops

On Thu, Jan 26, 2023 at 06:33:54PM +0100, Jules Maselbas wrote:

> @@ -58,9 +61,11 @@ static inline int generic_atomic_fetch_##op(int i, atomic_t *v)		\
>  static inline void generic_atomic_##op(int i, atomic_t *v)		\
>  {									\
>  	unsigned long flags;						\
> +	int c;								\
>  									\
>  	raw_local_irq_save(flags);					\
> -	v->counter = v->counter c_op i;					\
> +	c = arch_atomic_read(v);					\
> +	arch_atomic_set(v, c c_op i);					\
>  	raw_local_irq_restore(flags);					\
>  }

This and the others like it are a bit sad, it explicitly dis-allows the
compiler from using memops and forces a load-store.

The alternative is writing it like:

	*(volatile int *)&v->counter c_op i;

but that's pretty shit too I suppose.