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

[Florian Weimer]

* Chung-Lin Tang:

> On 2019/7/29 5:48 PM, Florian Weimer wrote:
>> * Chung-Lin Tang:
>
>>>> 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 wonder if it makes sense to disentangle this (desirable) functional
>> change from the rest (which sould be purely an optimization).
>
> By "functional change" here, are you referring to the testing framework,
> or the described ld.so destructor behavior I described above?

The destructor behavior.

>> Is it even necessary to re-sort on dlclose?  Is the original dependency
>> order available somewhere?  Then we could make it explicit that the
>> destructor order is the reverse of the constructor order (for the
>> objects unloaded).  Or is there a corner case which causes an expected
>> divergence?
>
> Dynamic loaded objects could add more relocation dependencies, and
> thus augment the dependency relations (by adding more constraints), so
> a final sort should still be required.

Yes, these dynamically added relocation dependencies could mean that
fewer objects than had been loaded by the dlopen can be freed with
dlclose.  But if we disregard those relocation dependencies for
destructor order sorting, wouldn't be the sorted result equivalent to
the constructor order?

Thanks,
Florian