Related question: Is the laziness done to save memory, startup time, or both? On Thu, Jun 21, 2018 at 7:36 AM Greg Clayton via Phabricator < revi...@reviews.llvm.org> wrote:
> clayborg added a comment. > > In https://reviews.llvm.org/D48393#1138989, @labath wrote: > > > I am not sure this will actually solve the problems you are seeing. This > may avoid corrupting the internal DenseMap data structures, but it will not > make the algorithm using them actually correct. > > For example the pattern in `ParseTypeFromDWARF` is: > > > > 1. check the "already parsed map". If the DIE is already parsed then > you're done. > > 2. if the map contains the magic "DIE_IS_BEING_PARSED" key, abort > (recursive dwarf references) > > 3. otherwise, insert the "DIE_IS_BEING_PARSED" key into the map > > 4. do the parsing, which potentially involves recursive > `ParseTypeFromDWARF` calls > > 5. insert the parsed type into the map > > > > What you do is make each of the steps (1), (3), (5) atomic > individually. However, the whole algorithm is not correct unless the whole > sequence is atomic as a whole. Otherwise, if you have two threads trying to > parse the same DIE (directly or indirectly), one of them could see the > intermediate DIE_IS_BEING_PARSED and incorrectly assume that it encountered > recursive types. > > > We need to make #1 atomic. > #2 would need to somehow know if the type is already being parsed > recursively by the current thread. If so, then do what we do now. If not, > we need a way to wait on the completion of this type so the other parsing > thread can complete it and put it into the map, at which time we grab the > right value from the map > So #6 step would need to be added so after we do put it into the map, we > can notify other threads that might be waiting > > > So, I think that locking at a higher level would be better. Doing that > will certainly be tricky though... > > > > > https://reviews.llvm.org/D48393 > > > >
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