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

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Message-ID: <772d8ec7-e743-4ea8-8d62-6acd80bdbc20@paulmck-laptop>
Date: Thu, 20 Feb 2025 17:26:56 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Marco Elver <elver@...gle.com>
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 Fri, Feb 21, 2025 at 01:16:00AM +0100, Marco Elver wrote:
> 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.

Fair enough!  Then would it work to just do "RCU" now, and ad the "BH"
and "IRQ" when those capabilities are added?

							Thanx, Paul