linux-kernel - Re: [PATCH] rtc: ds1685: actually spin forever in poweroff error path

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Message-ID: <20160307224415.GA8500@treble.redhat.com>
Date:	Mon, 7 Mar 2016 16:44:15 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Joshua Kinard <kumba@...too.org>
Cc:	Alessandro Zummo <a.zummo@...ertech.it>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	rtc-linux@...glegroups.com, linux-kernel@...r.kernel.org,
	kbuild test robot <fengguang.wu@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux/MIPS <linux-mips@...ux-mips.org>
Subject: Re: [PATCH] rtc: ds1685: actually spin forever in poweroff error path

On Mon, Mar 07, 2016 at 04:30:50PM -0500, Joshua Kinard wrote:
> On 03/07/2016 10:03, Josh Poimboeuf wrote:
> > objtool reports the following warnings:
> > 
> >   drivers/rtc/rtc-ds1685.o: warning: objtool: ds1685_rtc_work_queue()+0x0: duplicate frame pointer save
> >   drivers/rtc/rtc-ds1685.o: warning: objtool: ds1685_rtc_work_queue()+0x3: duplicate frame pointer setup
> >   drivers/rtc/rtc-ds1685.o: warning: objtool: ds1685_rtc_work_queue()+0x0: frame pointer state mismatch
> > 
> > The warning message needs to be improved, but what it really means in
> > this case is that ds1685_rtc_poweroff() has a possible code path where
> > it can actually fall through to the next function in the object code,
> > ds1685_rtc_work_queue().
> > 
> > The bug is caused by the use of the unreachable() macro in a place which
> > is actually reachable.  That causes gcc to assume that the printk()
> > immediately before the unreachable() macro never returns, when in fact
> > it does.  So gcc places the printk() at the very end of the function's
> > object code.  When the printk() returns, the next function starts
> > executing.
> > 
> > The surrounding comment and printk message state that the code should
> > spin forever, which explains the unreachable() statement.  However the
> > actual spin code is missing.
> 
> So this power down trick is used by both SGI O2 (IP32) and SGI Octane (IP30)
> systems via this RTC chip, and I've noticed lately that the Octane has stopped
> powering off via this function (it just sits and spins forever).  The O2 powers
> off as expected.  When I initially wrote this driver from the original version
> I found on LKML in '09, I hadn't gotten the Octane code back into a working
> shape, and once that happened, I only tested the non-SMP case (fixed Octane SMP
> in 4.1).  I suspect on the Octane, the use of SMP may be what is interfering
> somehow, and this bug may partially explain it.  This patch doesn't fix
> poweroff for me, but it's something to start from when I can get some time to
> chase it down.
> 
> That said, I initially left the 'while (1);' clause out because at one point
> during development, gcc yelled at me for using that at the end of the function,
> so I looked at some other drivers and saw the use of 'unreachable();' and used
> it instead.  Wasn't aware both of them are needed together in this instance.  I
> thought 'unreachable()' evaluated out to a 'while (1)' at the end.  Seems to
> actually be some kind of internal gcc trick.
> 
> How exactly did the kbuild bot trigger the above warnings?  I've only built and
> tested this driver on a MIPS platform and haven't seen that particular warning
> before.

Hi Joshua,

The warning was emitted by a brand new tool named objtool which does
some static object code analysis. It's currently only in linux-next, not
yet in Linus's tree.  To get the warning, you'd need to build the
linux-next tree for x86_64 with CONFIG_STACK_VALIDATION enabled.

Here's the kbuild bot warning:

  https://lkml.kernel.org/r/201603060005.PHCyifJr%fengguang.wu@intel.com


-- 
Josh