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Date:   Tue, 16 May 2023 17:52:03 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Akira Yokosawa <akiyks@gmail.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, linux-doc@vger.kernel.org, kernel-team@meta.com,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH locking/atomic 18/19] locking/atomic: Refrain from
 generating duplicate fallback kernel-doc
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Hi Paul,

On Sat, May 13, 2023 at 06:14:36PM -0700, Paul E. McKenney wrote:
> On Sun, May 14, 2023 at 08:58:00AM +0900, Akira Yokosawa wrote:
> > Running "./scripts/kernel-doc -none include/linux/atomic/atomic-arch-fallback.h"
> > on the tag emits a lot of warnings.
> > 
> > Looks like there are kernel-doc comments who don't have a corresponding
> > function signature next to them.
> > 
> >     /**
> >      * function_name() - Brief description of function.
> >      * @arg1: Describe the first argument.
> >      * @arg2: Describe the second argument.
> >      *        One can provide multiple line descriptions
> >      *        for arguments.
> >      *
> >      * A longer description, with more discussion of the function function_name()
> >      * that might be useful to those using or modifying it. Begins with an
> >      * empty comment line, and may include additional embedded empty
> >      * comment lines.
> >      */
> >     int function_name(int arg1, int arg2)  <---
> > 
> > Note that the kernel-doc script ignores #ifdef -- #else.
> 
> Me, I was thinking in terms of making this diagnostic ignore
> include/linux/atomic/atomic-arch-fallback.h.  ;-)
> 
> The actual definitions are off in architecture-specific files, and
> the kernel-doc headers could be left there.  But there are benefits to
> automatically generating all of them.
> 
> Another approach might be to put a "it is OK for the definition to
> be elsewhere" comment following those kernel-doc headers.
> 
> Any other ways to make this work?

I've spent the last day or so playing with this, and I think we can do this by
relegating the arch_atomic*() functions to an implementation detail (and not
documenting those with kerneldoc), and having a raw_atomic*() layer where we
flesh out the API, where each can have a mandatory function definition as
below:

/**
 * raw_atomic_fetch_inc_release() - does a thing atomically
 *
 * TODO: fill this in
 *
 * This is a version of atomic_fetch_inc_release() which is safe to use in
 * noinstr code. Unless instrumentation needs to be avoided,
 * atomic_fetch_inc_release() should be used in preference.
 */
static __always_inline int
raw_atomic_fetch_inc_release(atomic_t *v)
{
#if defined(arch_atomic_fetch_inc_release)
        return arch_atomic_fetch_inc_release(v)
#elif defined(arch_atomic_fetch_inc_relaxed)
        __atomic_release_fence();
        return arch_atomic_fetch_inc_relaxed(v);
#elif defined(arch_atomic_fetch_inc)
        return arch_atomic_fetch_inc(v)
#else   
        return raw_atomic_fetch_add_release(1, v);
#endif
}

... and likewise we can add comments for the regular instrumented atomics.

I've pushed out the WIP patches to my atomics/fallback-rework branch; if you're
happy to give me another day or two I can get a bit further.

> For me, the option of making this
> diagnostic ignore include/linux/atomic/atomic-arch-fallback.h has
> considerable attraction.

It's certainly appealing...

Thanks,
Mark.