linux-kernel - Re: [PATCH RFC 02/24] compiler-capability-analysis: Rename __cond_lock() to __cond

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Message-ID: <Z6XTKTo_LMj9KmbY@elver.google.com>
Date: Fri, 7 Feb 2025 10:32:25 +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 02/24] compiler-capability-analysis: Rename
 __cond_lock() to __cond_acquire()

On Fri, Feb 07, 2025 at 09:28AM +0100, Peter Zijlstra wrote:
> On Thu, Feb 06, 2025 at 07:09:56PM +0100, Marco Elver wrote:
> > Just like the pairing of attribute __acquires() with a matching
> > function-like macro __acquire(), the attribute __cond_acquires() should
> > have a matching function-like macro __cond_acquire().
> > 
> > To be consistent, rename __cond_lock() to __cond_acquire().
> 
> So I hate this __cond_lock() thing we have with a passion. I think it is
> one of the very worst annotations possible since it makes a trainwreck
> of the trylock code.
> 
> It is a major reason why mutex is not annotated with this nonsense.
> 
> Also, I think very dim of sparse in general -- I don't think I've ever
> managed to get a useful warning from between all the noise it generates.

Happy to reduce the use of __cond_lock(). :-)
Though one problem I found is it's still needed for those complex
statement-expression *_trylock that spinlock.h/rwlock.h has, where we
e.g. have (with my changes):

	#define raw_spin_trylock_irqsave(lock, flags)		\
		__cond_acquire(lock, ({				\
			local_irq_save(flags);			\
			_raw_spin_trylock(lock) ?		\
			1 : ({ local_irq_restore(flags); 0; }); \
		}))

Because there's an inner condition using _raw_spin_trylock() and the
result of _raw_spin_trylock() is no longer directly used in a branch
that also does the unlock, Clang becomes unhappy and complains. I.e.
annotating _raw_spin_trylock with __cond_acquires(1, lock) doesn't work
for this case because it's in a complex statement-expression. The only
way to make it work was to wrap it into a function that has attribute
__cond_acquires(1, lock) which is what I made __cond_lock/acquire do.

For some of the trivial uses, like e.g.

	#define raw_spin_trylock(lock)	__cond_acquire(lock, _raw_spin_trylock(lock))

it's easy enough to remove the outer __cond_lock/acquire if e.g. the
_raw_spin_trylock has the attribute __cond_acquires. I kept these around
for Sparse compatibility, but if we want to get rid of Sparse
compatibility, some of those can be simplified.