Re: [PATCH v4 00/18] All-stop on top of non-stop

Luis Machado
2015-08-12T20:18:40Z

On 08/12/2015 04:59 PM, Joel Brobecker wrote:
>> Two things about the patch. I see that the change to GDB's code is almost
>> trivial, but the testcase looks quite involved.
>>
>> The first thing is that one wouldn't be able to tell what the testcase does
>> without looking at the commit log. dso2dso doesn't particularly translate to
>> "displaced stepping instruction masking problem on amd64". Should we change
>> the testcase name to something a bit more meaningful? Maybe document it a
>> bit?
>>
>> The second point is, should we restrict this testcase to be executed only
>> for amd64? Maybe move it to gdb.arch? Unless you actually tried this
>> testcase with different architectures and confirmed the testcase is sane
>> there, it feels a bit iffy to add/execute this for non-amd64 targets.
>>
>> In the worst case, we could have a failure due to different instruction
>> scheduling schemes (or maybe even a displaced stepping bug) on non-amd64
>> targets. At a minimum, we'd have a spurious PASS that wouldn't be meaningful
>> for the overall test results since things never failed on said architecture
>> in the first place.
>>
>> Alternatively, for such a trivial fix, shouldn't we go without a testcase?
>
> Some good points. To me, the purpose of the test is to verify that
> stepping from some code in one DSO over a call to a function in another
> DSO, is working on all platforms that support shared libraries. That's
> the main purpose of the test. If, one day, the implementation changes
> so that displaced stepping is no longer necessary, then what matters
> at the user level is that this testcase continues to behave the same.

Ok. So i think i misunderstood the purpose of the testcase there. In
reality the testcase is not testing the fix itself, but rather
introducing a new test not related to the problem, except in the amd64
architecture, where it really tests the problem.

It just confused me that the test is generic for other non-amd64
architectures and specific to the amd64 problem you saw.

I'd expect a generic solib test to be included in one of our shared
library tests, but then you'd have to shape it in a way that would
exercise your displaced stepping problem.

>
> Even if you wanted a gdb.arch test that verified this specific bug,
> you'd have to find a way for the test to load at an address that's
> high enough that the mask starts causing problems. I don't think
> that's very easy, and I'm not sure it's worth our time.
>
> Regarding the complexity of the test, I think we need to try it and
> see. At AdaCore, we run the equivalent Ada testcase nightly on Linux
> (x86, x86_64 and ppc), Windows (x86 and x86_64), Solaris (sparc) and
> Darwin (x86_64), and it passes everywhere. It passes with GDBserver
> too. My take would be that we'd be fixing trivial errors in the testcase
> itself, and just accept the failures on platforms where it reveals
> a bona fide issue, as we've beeing doing with all the other tests.

It is not a big deal. In recent times i've been seeing a number of
testcases that shouldn't be generally executed for all architectures.
That ends up causing spurious failures and bringing the signal/noise
ratio of the testsuite down for the affect architectures (usually not
x86 or Linux).

I see your targets are mostly x86. I can give it a try on a few more
(powerpc, mips, arm, nios) and let you know what i see. How does that sound?