Re: [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal locks

Waiman Long
2018-11-12T22:56:13Z

On 11/12/2018 02:22 PM, Josh Poimboeuf wrote:
> On Mon, Nov 12, 2018 at 07:30:50AM +0100, Ingo Molnar wrote:
>> * Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>>
>>> On Mon, Nov 12, 2018 at 06:10:33AM +0100, Ingo Molnar wrote:
>>>> * Waiman Long <longman@redhat.com> wrote:
>>>>
>>>>> On 11/10/2018 09:10 AM, Peter Zijlstra wrote:
>>>>>> On Fri, Nov 09, 2018 at 09:04:12AM +0100, Ingo Molnar wrote:
>>>>>>> BTW., if you are interested in more radical approaches to optimize
>>>>>>> lockdep, we could also add a static checker via objtool driven call graph
>>>>>>> analysis, and mark those locks terminal that we can prove are terminal.
>>>>>>>
>>>>>>> This would require the unified call graph of the kernel image and of all
>>>>>>> modules to be examined in a final pass, but that's within the principal
>>>>>>> scope of objtool. (This 'final pass' could also be done during bootup, at
>>>>>>> least in initial versions.)
>>>>>> Something like this is needed for objtool LTO support as well. I just
>>>>>> dread the build time 'regressions' this will introduce :/
>>>>>>
>>>>>> The final link pass is already by far the most expensive part (as
>>>>>> measured in wall-time) of building a kernel, adding more work there
>>>>>> would really suck :/
>>>>> I think the idea is to make objtool have the capability to do that. It
>>>>> doesn't mean we need to turn it on by default in every build.
>>>> Yeah.
>>>>
>>>> Also note that much of the objtool legwork would be on a per file basis
>>>> which is reasonably parallelized already. On x86 it's also already done
>>>> for every ORC build i.e. every distro build and the incremental overhead
>>>> from also extracting locking dependencies should be reasonably small.
>>>>
>>>> The final search of the global graph would be serialized but still
>>>> reasonably fast as these are all 'class' level dependencies which are
>>>> much less numerous than runtime dependencies.
>>>>
>>>> I.e. I think we are talking about tens of thousands of dependencies, not
>>>> tens of millions.
>>>>
>>>> At least in theory. ;-)
>>> Generating a unified call graph sounds very expensive (and very far
>>> beyond what objtool can do today).
>> Well, objtool already goes through the instruction stream and recognizes
>> function calls - so it can in effect generate a stream of "function x
>> called by function y" data, correct?
> Yeah, though it would be quite simple to get the same data with a simple
> awk script at link time.
>
>>> Also, what about function pointers?
>> So maybe it's possible to enumerate all potential values for function
>> pointers with a reasonably simple compiler plugin and work from there?
> I think this would be somewhere between very difficult and impossible to
> do properly. I can't even imagine how this would be implemented in a
> compiler plugin. But I'd love to be proven wrong on that.

I would say we have to assume for the worst when a function pointer is
being called while holding a lock unless we are able to find out all its
possible targets.

Cheers,
Longman