Re: [Xen-devel] [PATCH v2 0/6] Remove dependency on __LINE__

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.03.17 at 09:29, <ross.lagerwall@xxxxxxxxxx> wrote:
> On 03/09/2017 10:34 AM, Jan Beulich wrote:
>>>>> On 08.03.17 at 18:46, <ross.lagerwall@xxxxxxxxxx> wrote:
>>> Sorry for the long delay since the first version of this series
>>> (previously called "Make building xSplice patches easier").  Here is a
>>> set of changes that remove the use of __LINE__ when building with NDEBUG
>>> and LivePatch enabled.  Tested to boot on x86.  Compile-tested on arm.
>>
>> What I'm missing here is an evaluation of possible alternatives, as
>> converting from __LINE__ to code addresses implies an extra step
>> when wanting to look up the place where a problem occurred,
>> which I generally consider undesirable. For example, I had briefly
>> discussed with Andrew the addition of #line directives, but iirc he
>> had indicated some issue with that. Furthermore there is the
>> option (at least for some of the more simple patches) of squeezing
>> more than one statement on a single line, perhaps just for patch
>> creation. And I guess one can think of more.
>>
> 
> Both of the options you suggested require extra work when building a 
> live patch. This series is intended to remove that extra work as the 
> process is already more difficult than I'd like.
> 
> I don't think using addr2line is much harder than looking up the line 
> number directly, especially as it is something that needs to be done for 
> many other crashes.

I agree with the second half, but I don't think I do for the first part:
I don't know how you would manage this, but for me this is several
steps:
- Locate and download the correct package (it's not reasonable to
  keep them all, there simply are too many)
- extract xen-syms or xen.efi (and of course I will need to know
  which one was actually used)
- run addr2line on it
Quite a bit more than just going to the source file and finding the
line right away - in most cases line numbers, as opposed to code
addresses, don't change much despite a delta of a few dozen
backports or other patches, so normally one doesn't even need
to look at the _precise_ tree.

> On your suggestion, use of __LINE__ is disabled only 
> when CONFIG_LIVEPATCH=y and NDEBUG which means that in a typical 
> development scenario, you'd still get line numbers.

That's understood, but I'm concerned about extra overhead when
looking at customer reports.

> They would be 
> removed only for "release" builds in which it is likely that the source 
> code & debuginfo is archived somewhere such that looking up a line 
> number requires several steps anyway. I could suggest making it a 
> separate config option but IIRC you prefer to limit the number of config 
> options.

I prefer to limit ones with non-EXPERT-visible prompts, yes. But
I also don't see how a separate config option would improve the
situation.

Jan


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