linux-kernel - Re: [PATCH v1 1/1] platform/x86: ideapad-laptop: Make the scope

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Message-ID: <Ztct4P_PvQmeq_ih@smile.fi.intel.com>
Date: Tue, 3 Sep 2024 18:40:16 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Gergo Koteles <soyer@....hu>
Cc: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	Hans de Goede <hdegoede@...hat.com>,
	platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ike Panhc <ike.pan@...onical.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v1 1/1] platform/x86: ideapad-laptop: Make the
 scope_guard() clear of its scope

On Tue, Sep 03, 2024 at 05:29:02PM +0200, Gergo Koteles wrote:
> On Tue, 2024-09-03 at 18:14 +0300, Andy Shevchenko wrote:
> > On Tue, Sep 03, 2024 at 05:00:51PM +0200, Gergo Koteles wrote:
> > > On Thu, 2024-08-29 at 19:50 +0300, Andy Shevchenko wrote:
> > > > First of all, it's a bit counterintuitive to have something like
> > > > 
> > > > 	int err;
> > > > 	...
> > > > 	scoped_guard(...)
> > > > 		err = foo(...);
> > > > 	if (err)
> > > > 		return err;
> > > > 
> > > > Second, with a particular kernel configuration and compiler version in
> > > > one of such cases the objtool is not happy:
> > > > 
> > > >   ideapad-laptop.o: warning: objtool: .text.fan_mode_show: unexpected end of section
> > > > 
> > > > I'm not an expert on all this, but the theory is that compiler and
> > > > linker in this case can't understand that 'result' variable will be
> > > > always initialized as long as no error has been returned. Assigning
> > > > 'result' to a dummy value helps with this. Note, that fixing the
> > > > scoped_guard() scope (as per above) does not make issue gone.
> > > > 
> > > > That said, assign dummy value and make the scope_guard() clear of its scope.
> > > > For the sake of consistency do it in the entire file.
> > > > 
> > > 
> > > Interestingly, if I open a scope manually and use the plain guard, the
> > > warning disappears.
> > 
> > Yes, that's what I also have, but I avoid that approach because in that case
> > the printing will be done inside the lock, widening the critical section for
> > no benefits.
> > 
> 
> This is intended to be an inner block scope within the function, it
> does not expand the critical section.

I'm not sure I understand.

scoped_guard() has a marked scope (with {} or just a line coupled with it).
The guard() has a scope starting at it till the end of the function. In the
latter case the sysfs_emit() becomes part of the critical section.

> > > 	unsigned long result;
> > > 	int err;
> > > 
> > > 	{
> > > 		guard(mutex)(&priv->vpc_mutex);
> > > 		err = read_ec_data(priv->adev->handle, VPCCMD_R_FAN,
> > > &result);
> > > 		if (err)
> > > 			return err;
> > > 	}

But looking again into the code above now I got what you meant.
You have added a nested scope inside the function, like

	do {
		...
	} while (0);

Yes, this is strange and not what we want to have either. So I prefer to hear
what objtool / clang people may comment on this.

Sorry that I missed this.

> > > This looks a bit strange, but is probably easier for the compiler than
> > > the for loop of scoped_guard.
> > > 
> > > But I don't know how well this style fits into the kernel.

-- 
With Best Regards,
Andy Shevchenko