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

Vineet Gupta
2017-12-20T18:52:44Z

On 12/20/2017 01:01 AM, Arnd Bergmann wrote:
> On Tue, Dec 19, 2017 at 11:38 PM, Vineet Gupta
> <Vineet.Gupta1@synopsys.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@synopsys.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.

Sorry, I didn't realize we are missing the stack trace now which you removed from
the patch - why ? Did u intend to reduce inline generated code for the stack dump
calls - which sounds like a great idea. But it would only work for the synchronous
abort() but not when builtin translates to actual trap inducing instruction.

> 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?

Indeed so. I didn't run into this in my testing as my for-curr has an ARC specific
version (predating Sudip's generic version- because of build failures in our
internal regression jobs etc). That version only calls panic.

abort() is only likely to be called due to __builtin_trap() for arches where gcc
doesn't have a target specific defn of it. And thus adding the call from BUG()
will cause the recursion as you found out with Sudip's generic version and thus
needs a fixup.

Thx,
-Vineet