linux-kernel - Re: [PATCH v5 15/36] srcu: Support Clang's context analysis

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Message-ID: <CANpmjNPs9CtY1w1-MqL1-CnHVFLxXoA2rbd6d2w4wfxT8AP0ew@mail.gmail.com>
Date: Tue, 27 Jan 2026 00:46:49 +0100
From: Marco Elver <elver@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Bart Van Assche <bvanassche@....org>, Boqun Feng <boqun.feng@...il.com>, 
	Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>, 
	"David S. Miller" <davem@...emloft.net>, Luc Van Oostenryck <luc.vanoostenryck@...il.com>, 
	Chris Li <sparse@...isli.org>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Alexander Potapenko <glider@...gle.com>, Arnd Bergmann <arnd@...db.de>, Christoph Hellwig <hch@....de>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Eric Dumazet <edumazet@...gle.com>, 
	Frederic Weisbecker <frederic@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Herbert Xu <herbert@...dor.apana.org.au>, Ian Rogers <irogers@...gle.com>, 
	Jann Horn <jannh@...gle.com>, Joel Fernandes <joelagnelf@...dia.com>, 
	Johannes Berg <johannes.berg@...el.com>, Jonathan Corbet <corbet@....net>, 
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Subject: Re: [PATCH v5 15/36] srcu: Support Clang's context analysis

On Mon, 26 Jan 2026 at 22:36, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Jan 26, 2026 at 10:54:56AM -0800, Bart Van Assche wrote:
>
> > Has it ever been considered to add support in the clang compiler for a
> > variant of __must_hold() that expresses that one of two capabilities
> > must be held by the caller? I think that would remove the need to
> > annotate SRCU update-side code with __acquire_shared(ssp) and
> > __release_shared(ssp).
>
> Right, I think I've asked for logical operators like that. Although I
> think it was in the __guarded_by() clause rather than the __must_hold().
> Both || and && would be nice to have ;-)

Some attributes take multiple arguments (__must_hold does), though
__guarded_by doesn't. Yet, && can still be had with adding it multiple
times e.g. '__guarded_by(pi_lock) __guarded_by(rq->__lock)'.

Only thing that doesn't exist is ||. I think the syntax you ask for
won't fly, but I can add it to the backlog to investigate an _any
variant of these attributes. Don't hold your breath though, given the
time it takes to land all that in a released Clang version.

> Specifically, I think I asked for something like:
>
>         cpumask_t       cpus_allowed __guarded_by(pi_lock && rq->__lock)
>                                      __guarded_shared_by(pi_lock || rq->__lock);
>
>
> I think Marco's suggestion was to use 'fake' locks to mimic those
> semantics.