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

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Date:   Wed, 20 Dec 2017 10:01:27 +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 11:38 PM, Vineet Gupta
<Vineet.Gupta1@...opsys.com> wrote:
> 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>

I meant whether it prints the right registers and stack trace, but I
assume you tested that and just did not list it above.

> 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 !

Hmm, so with the new definition of abort(),

+__weak void abort(void)
+{
+   BUG();
+
+   /* if that doesn't kill us, halt */
+   panic("Oops failed to kill thread");
+}

won't that run into an endless recursion? Or do you then override abort()
for ARC?

         Arnd