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

Ingo Molnar
2018-11-12T06:30:58Z

* 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?

> 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?

One complication would be function pointers encoded as opaque data
types...

> BTW there's another kernel static analysis tool which attempts to
> create such a call graph already: smatch.

It's not included in the kernel tree though and I'd expect tight coupling
(or at least lock-step improvements) between tooling and lockdep here.

Thanks,

Ingo