linux-kernel - Re: [RFC PATCH v2 11/31] kvx: Add atomic/locking headers

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Date:   Thu, 26 Jan 2023 11:15:37 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Jules Maselbas <jmaselbas@...ray.eu>
Cc:     Yann Sionneau <ysionneau@...ray.eu>, Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <maz@...nel.org>,
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        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Will Deacon <will@...nel.org>,
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        Oleg Nesterov <oleg@...hat.com>,
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        Waiman Long <longman@...hat.com>,
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Subject: Re: [RFC PATCH v2 11/31] kvx: Add atomic/locking headers

Hi Jules,

On Thu, Jan 26, 2023 at 10:57:20AM +0100, Jules Maselbas wrote:
> Hi Mark,
> 
> On Fri, Jan 20, 2023 at 03:18:48PM +0000, Mark Rutland wrote:
> > On Fri, Jan 20, 2023 at 03:09:42PM +0100, Yann Sionneau wrote:
> > > +#define ATOMIC64_RETURN_OP(op, c_op)					\
> > > +static inline long arch_atomic64_##op##_return(long i, atomic64_t *v)	\
> > > +{									\
> > > +	long new, old, ret;						\
> > > +									\
> > > +	do {								\
> > > +		old = v->counter;					\
> > 
> > This should be arch_atomic64_read(v), in order to avoid the potential for the
> > compiler to replay the access and introduce ABA races and other such problems.
> Thanks for the suggestion, this will be into v3.
> 
> > For details, see:
> > 
> >   https://lore.kernel.org/lkml/Y70SWXHDmOc3RhMd@osiris/
> >   https://lore.kernel.org/lkml/Y71LoCIl+IFdy9D8@FVFF77S0Q05N/
> > 
> > I see that the generic 32-bit atomic code suffers from that issue, and we
> > should fix it.
> I took a look at the generic 32-bit atomic, but I am unsure if this
> needs to be done for both the SMP and non-SMP implementations. But I
> can send a first patch and we can discuss from there.

Sounds good to me; thanks!

[...]

> > > +static inline int arch_atomic_add_return(int i, atomic_t *v)
> > > +{
> > > +	int new, old, ret;
> > > +
> > > +	do {
> > > +		old = v->counter;
> > 
> > Likewise, arch_atomic64_read(v) here.
> ack, this will bt arch_atomic_read(v) here since this is not atomic64_t
> here.

Ah, yes, my bad!

Thanks,
Mark.