linux-kernel - Re: [RFC PATCH] cleanup: make scoped

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Message-ID: <faff2ffd-d36b-4655-80dc-35f772748a6c@stanley.mountain>
Date: Fri, 27 Sep 2024 18:04:45 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Przemek Kitszel <przemyslaw.kitszel@...el.com>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
	amadeuszx.slawinski@...ux.intel.com,
	Tony Nguyen <anthony.l.nguyen@...el.com>,
	nex.sw.ncis.osdt.itp.upstreaming@...el.com, netdev@...r.kernel.org,
	Andy Shevchenko <andriy.shevchenko@...el.com>
Subject: Re: [RFC PATCH] cleanup: make scoped_guard() to be return-friendly

On Fri, Sep 27, 2024 at 04:08:30PM +0200, Przemek Kitszel wrote:
> On 9/27/24 09:31, Dan Carpenter wrote:
> > On Thu, Sep 26, 2024 at 03:41:38PM +0200, Przemek Kitszel wrote:
> > > diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> > > index d9e613803df1..6b568a8a7f9c 100644
> > > --- a/include/linux/cleanup.h
> > > +++ b/include/linux/cleanup.h
> > > @@ -168,9 +168,16 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
> > >   #define __guard_ptr(_name) class_##_name##_lock_ptr
> > > -#define scoped_guard(_name, args...)					\
> > > -	for (CLASS(_name, scope)(args),					\
> > > -	     *done = NULL; __guard_ptr(_name)(&scope) && !done; done = (void *)1)
> > > +#define scoped_guard(_name, args...)	\
> > > +	__scoped_guard_labeled(__UNIQUE_ID(label), _name, args)
> > > +
> > > +#define __scoped_guard_labeled(_label, _name, args...)	\
> > > +	if (0)						\
> > > +		_label: ;				\
> > > +	else						\
> > > +		for (CLASS(_name, scope)(args);		\
> > > +		     __guard_ptr(_name)(&scope), 1;	\
> >                                                 ^^^
> > > +		     ({ goto _label; }))
> > 
> > Remove the ", 1".  The point of the __guard_ptr() condition is for try_locks
> > but the ", 1" means they always succeed.  The only try lock I can find in
> 
> You are right that the __guard_ptr() is conditional for the benefit of
> try_locks. But here we have unconditional lock. And removing ", 1" part
> makes compiler complaining with the very same message:
> error: control reaches end of non-void function [-Werror=return-type]
> 
> so ", 1" part is on purpose and must stay there to aid compiler.
> 
> > the current tree is tsc200x_esd_work().

Obviously, we can't break stuff and also checking __guard_ptr(_name)(&scope) is
pointless if we're going to ignore the return value.

But, sure, I get that we want to the compiler to know that regular spin_lock()
is going to succeed and spin_trylock() might not.  As a static checker
developer, I want that as well.  Currently, whenever someone creates a new class
of locks, I have to add a couple lines to Smatch to add this information.  It's
not a huge deal, but it would be nice to avoid this.

I did a `git grep scoped_guard | grep try` and I think tsc200x_esd_work() is the
only place which actually uses try locks with scoped_guard().  If it's just the
one, then why don't we create a scoped_guard_trylock() macro?

regards,
dan carpenter