Re: [PATCH 1/2][RFC] #17645, fix slow DSO sorting behavior in dynamic loader

[Chung-Lin Tang <chunglin_tang at mentor dot com>]

On 2019/7/23 9:21 PM, Florian Weimer wrote:
> Is => intended to cover the case of run-time dependencies added late due
> to lazy binding?
>
> Currently, those late dependencies have two effects, I think: They keep
> around the referenced libraries longer than before (so that dlclose
> would not remove an object which is still in used solely due to lazy
> binding).  And the ELF destructors are reordered to reflect these added
> run-time dependencies.

Yes, you can test that. The effect of => is to create a caller/callee relation
between objects: 'x=>y' creates fn_x() and fn_y() in those two DSOs,
and fn_x() has a call to fn_y().

Though that's the only immediate effect that => has. To construct a test of
run-time added dependencies related to dlopen/etc. you also need to add those
operations inside the '{}' construct.

All the created DSOs have a constructor/destructor that outputs their single
character name. The generated main() program prints '[]' brackets after
dlopen/dlclose calls to separate out the following constructor/destructor output.
So taken whole, the entire output string should capture all constructor/destructor
activity and ordering behavior.

> Can your test framework test both cases?  What's your position on the
> second effect?  I think it sometimes results in destructors running not
> in the opposite order of constructors, due to the new topological sort.
> (This also happens with the current implementation.)

What I did in the ld.so code patch was add a second pass of sorting that ignores
runtime deps, prioritizing link dependencies; this appears to also be what prior
discussion pointed towards, see more details in that 2nd email with the actual
code patch.

I have attached an updated patch here; fixed some bugs in the script related to
the '@' operator for the main program construct.

Thanks,
Chung-Lin

Attachment: dlsort-01-tests-v2.patch
Description: Text document