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

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Message-ID: <f31caa9b-89f0-4168-04bf-14a98595865e@synopsys.com>
Date:   Tue, 19 Dec 2017 14:38:21 -0800
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Arnd Bergmann <arnd@...db.de>
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>,
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        "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 12/19/2017 12:13 PM, Arnd Bergmann wrote:
>
>> 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();

It does, added a BUG() arbit, hits an abort()

...
ISA Extn    : atomic ll64 unalign (not used)
         : mpy[opt 9] div_rem norm barrel-shift swap minmax swape
BPU        : partial match, cache:2048, Predict Table:16384
BUG: failure at ../arch/arc/mm/tlb.c:827/arc_mmu_init()!


Tested-by: Vineet Gupta <vgupta@...opsys.com>

FWIW newer ARC gcc actually implements the builtin so we get a trap 5 instruction 
now, vs., abort() calls before.

BTW I missed reading the hunk of your changelog where this addresses the long 
standing mystery with ARC builds and numerous -Wreturn-type warnings. I always 
wondered why they were not fixed upstream already, being too lazy to investigate 
myself, and turns out this was due to this BUG() thingy. phew !

-Vineet