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

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Date:   Fri, 27 Jan 2023 14:49:46 +0100
From:   Jules Maselbas <jmaselbas@...ray.eu>
To:     Peter Zijlstra <peterz@...radead.org>
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

Hi Peter,

On Fri, Jan 27, 2023 at 12:18:13PM +0100, Peter Zijlstra wrote:
> 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.
Good point, I don't know much about atomic memops but this is indeed a
bit sad to prevent such instructions to be used.

> The alternative is writing it like:
> 
> 	*(volatile int *)&v->counter c_op i;
I wonder if it could be possible to write something like:

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

I also noticed that GCC has some builtin/extension to do such things,
__atomic_OP_fetch and __atomic_fetch_OP, but I do not know if this
can be used in the kernel.


Thanks,
-- Jules