linux-kernel - Re: [PATCH] bug.h: Work around GCC PR82365 in BUG()

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Date:   Tue, 19 Dec 2017 21:13:12 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>,
        Mikael Starvik <starvik@...s.com>,
        Jesper Nilsson <jesper.nilsson@...s.com>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "David S. Miller" <davem@...emloft.net>,
        Christopher Li <sparse@...isli.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will.deacon@....com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Mark Rutland <mark.rutland@....com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-cris-kernel@...s.com" <linux-cris-kernel@...s.com>,
        "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
        "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-sparse@...r.kernel.org" <linux-sparse@...r.kernel.org>
Subject: Re: [PATCH] bug.h: Work around GCC PR82365 in BUG()

On Tue, Dec 19, 2017 at 5:57 PM, Vineet Gupta
<Vineet.Gupta1@...opsys.com> wrote:
> On 12/19/2017 03:41 AM, Arnd Bergmann wrote:

>> In case of ARC and CRIS, it turns out that the BUG() implementation
>> actually does return (or at least the compiler thinks it does), resulting
>> in lots of warnings about uninitialized variable use and leaving noreturn
>> functions, such as:
>>
>> block/cfq-iosched.c: In function 'cfq_async_queue_prio':
>> block/cfq-iosched.c:3804:1: error: control reaches end of non-void
>> function [-Werror=return-type]
>> include/linux/dmaengine.h: In function 'dma_maxpq':
>> include/linux/dmaengine.h:1123:1: error: control reaches end of non-void
>> function [-Werror=return-type]

>> diff --git a/arch/arc/include/asm/bug.h b/arch/arc/include/asm/bug.h
>> index ea022d47896c..21ec82466d62 100644
>> --- a/arch/arc/include/asm/bug.h
>> +++ b/arch/arc/include/asm/bug.h
>> @@ -23,7 +23,8 @@ void die(const char *str, struct pt_regs *regs, unsigned
>> long address);
>>     #define BUG()       do {
>> \
>>         pr_warn("BUG: failure at %s:%d/%s()!\n", __FILE__, __LINE__,
>> __func__); \
>> -       dump_stack();
>> \
>> +       barrier_before_unreachable();
>> \
>> +       __builtin_trap();
>> \
>>   } while (0)
>>     #define HAVE_ARCH_BUG
>
>
> I suppose BUG() implies "dead end" like semantics - which ARC was lacking
> before ?

Correct. Using __builtin_trap() here avoids the 'control reaches end of non-void
function' warnings, but then makes us run into the stack size problem that
I work around with the barrier_before_unreachable().

It would be good if you could give this a quick test to see if you get sensible
output from the __builtin_trap();

         Arnd