linux-kernel - Re: [PATCH RFC 15/24] rcu: Support Clang's capability analysis

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Message-ID: <CANpmjNOreC6EqOntBEOAVZJ5QuSnftoa0bc7mopeMt76Bzs1Ag@mail.gmail.com>
Date: Fri, 21 Feb 2025 20:46:45 +0100
From: Marco Elver <elver@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, Alexander Potapenko <glider@...gle.com>, 
	Bart Van Assche <bvanassche@....org>, Bill Wendling <morbo@...gle.com>, Boqun Feng <boqun.feng@...il.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Frederic Weisbecker <frederic@...nel.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Ingo Molnar <mingo@...nel.org>, 
	Jann Horn <jannh@...gle.com>, Joel Fernandes <joel@...lfernandes.org>, 
	Jonathan Corbet <corbet@....net>, Josh Triplett <josh@...htriplett.org>, 
	Justin Stitt <justinstitt@...gle.com>, Kees Cook <kees@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Miguel Ojeda <ojeda@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Uladzislau Rezki <urezki@...il.com>, Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>, 
	kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev, rcu@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH RFC 15/24] rcu: Support Clang's capability analysis

On Fri, 21 Feb 2025 at 19:52, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Feb 21, 2025 at 10:08:06AM -0800, Paul E. McKenney wrote:
>
> > > ... unfortunately even for shared locks, the compiler does not like
> > > re-entrancy yet. It's not yet supported, and to fix that I'd have to go
> > > and implement that in Clang first before coming back to this.
> >
> > This would be needed for some types of reader-writer locks, and also for
> > reference counting, so here is hoping that such support is forthcoming
> > sooner rather than later.
>
> Right, so I read the clang documentation for this feature the other day,
> and my take away was that this was all really primitive and lots of work
> will need to go into making this more capable before we can cover much
> of the more interesting things we do in the kernel.
>
> Notably the whole guarded_by member annotations, which are very cool in
> concept, are very primitive in practise and will need much extensions.

I have one extension in flight:
https://github.com/llvm/llvm-project/pull/127396 - it'll improve
coverage for pointer passing of guarded_by members.

Anything else you see as urgent? Re-entrant locks support a deal breaker?

But yes, a lot of complex locking patterns will not easily be
expressible right away.

> To that effect, and because this is basically a static analysis pass
> with no codegen implications, I would suggest that we keep the whole
> feature limited to the very latest clang version for now and don't
> bother supporting older versions at all.

Along those lines, in an upcoming v2, I'm planning to bump it up to
Clang 20+ because that version introduced a reasonable way to ignore
warnings in not-yet-annotated headers:
https://git.kernel.org/pub/scm/linux/kernel/git/melver/linux.git/commit/?h=cap-analysis/dev&id=2432a39eae8197f5058c578430bd1906c18480c3