linux-kernel - Re: [PATCH v2] atomics: fix atomic64_{read_acquire,set

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Message-Id: <b79c521a-f857-41fa-8bba-ee013f5e4fa8@www.fastmail.com>
Date:   Fri, 04 Feb 2022 23:45:47 +0800
From:   "Boqun Feng" <boqun.feng@...il.com>
To:     "Mark Rutland" <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, ardb@...nel.org, arnd@...db.de,
        "Peter Zijlstra" <peterz@...radead.org>, will@...nel.org
Subject: Re: [PATCH v2] atomics: fix atomic64_{read_acquire,set_release} fallbacks



On Fri, Feb 4, 2022, at 7:16 PM, Mark Rutland wrote:
> On Fri, Feb 04, 2022 at 08:20:25AM +0800, Boqun Feng wrote:
>> On Thu, Feb 03, 2022 at 04:12:43PM +0000, Mark Rutland wrote:
>> > diff --git a/include/linux/atomic/atomic-arch-fallback.h b/include/linux/atomic/atomic-arch-fallback.h
>> > index a3dba31df01e..6db58d180866 100644
>> > --- a/include/linux/atomic/atomic-arch-fallback.h
>> > +++ b/include/linux/atomic/atomic-arch-fallback.h
>> > @@ -151,7 +151,16 @@
>> >  static __always_inline int
>> >  arch_atomic_read_acquire(const atomic_t *v)
>> >  {
>> > -	return smp_load_acquire(&(v)->counter);
>> > +	int ret;
>> > +
>> > +	if (__native_word(atomic_t)) {
>> > +		ret = smp_load_acquire(&(v)->counter);
>> > +	} else {
>> > +		ret = arch_atomic_read(v);
>> > +		__atomic_acquire_fence();
>> > +	}
>> > +
>> > +	return ret;
>> >  }
>> >  #define arch_atomic_read_acquire arch_atomic_read_acquire
>> >  #endif
>> > @@ -160,7 +169,12 @@ arch_atomic_read_acquire(const atomic_t *v)
>> >  static __always_inline void
>> >  arch_atomic_set_release(atomic_t *v, int i)
>> >  {
>> > -	smp_store_release(&(v)->counter, i);
>> > +	if (__native_word(atomic_t)) {
>> > +		smp_store_release(&(v)->counter, i);
>> > +	} else {
>> > +		__atomic_release_fence();
>> > +		arch_atomic_set(v, i);
>> > +	}
>> >  }
>> 
>> The changes for atomic_t are not necessary, right? They are correct but
>> "side effects" because of the change in scripts.
>
> Correct -- those aren't necessary, but aren't harmful, and it's simpler to have
> than than it is to special-case the scripts.
>
>> If so, it's better to mention this somewhere.
>
> Sure; how about I add the following to the commit message:
>
> | Since the fallback templates are used to generate the atomic64_*() and
> | atomic_*() operations, the __native_word() check is added to both. For
> | the atomic_*() operations, which are always 32-bit, the __native_word()
> | check is redundant but not harmful, as it is always true.
>
> ... ?
>

Looks good to me!

Regards,
Boqun

> Thanks,
> Mark.