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

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Date:   Fri, 27 Jan 2023 15:34:33 +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 Fri, Jan 27, 2023 at 02:49:46PM +0100, Jules Maselbas wrote:
> 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.

Depends on the platform, x86,s390 etc. have then, RISC like things
typically don't.

> > 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;

Should work, but give it a try, see what it does :-)

> 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.

On a per-architecture basis only, the C/C++ memory model does not match
the Linux Kernel memory model so using the compiler to generate the
atomic ops is somewhat tricky and needs architecture audits.