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

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Message-ID: <CANpmjNNHTg+uLOe-LaT-5OFP+bHaNxnKUskXqVricTbAppm-Dw@mail.gmail.com>
Date: Fri, 21 Feb 2025 01:16:00 +0100
From: Marco Elver <elver@...gle.com>
To: paulmck@...nel.org
Cc: 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>, 
	Peter Zijlstra <peterz@...radead.org>, 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 Thu, 20 Feb 2025 at 23:36, Paul E. McKenney <paulmck@...nel.org> wrote:
[...]
> Suppose that one function walks an RCU-protected list, calling some
> function from some other subsystem on each element.  Suppose that each
> element has another RCU protected list.
>
> It would be good if the two subsystems could just choose their desired
> flavor of RCU reader, without having to know about each other.

That's what I figured might be the case - thanks for clarifying.

> > Another problem was that if we want to indicate that "RCU" read lock
> > is held, then we should just be able to write
> > "__must_hold_shared(RCU)", and it shouldn't matter if rcu_read_lock()
> > or rcu_read_lock_bh() was used. Previously each of them acquired their
> > own capability "RCU" and "RCU_BH" respectively. But rather, we're
> > dealing with one acquiring a superset of the other, and expressing
> > that is also what I attempted to solve.
> > Let me rethink this...
>
> Would it work to have just one sort of RCU reader, relying on a separate
> BH-disable capability for the additional semantics of rcu_read_lock_bh()?

That's what I've tried with this patch (rcu_read_lock_bh() also
acquires "RCU", on top of "RCU_BH"). I need to add a re-entrancy test,
and make sure it doesn't complain about that. At a later stage we
might also want to add more general "BH" and "IRQ" capabilities to
denote they're disabled when held, but that'd overcomplicate the first
version of this series.

Thanks,
-- Marco