[llvm-dev] LLVMContext: Threads and Ownership.

[Lang Hames via llvm-dev]

>
> I'd think/suggest ref-counted LLVMContext ownership would be done by
> wrapping/external tracking in this use case.


What kind of wrapping are you imagining? I'm worried that maintaining an
external context ref-count is going to be awkward and error prone, but
perhaps there are idioms that would make this easier than I am imagining.

I think it may be simpler and safer to expose the number of Modules managed
by an LLVMContext. That way the "ref-count" always reflects the number of
modules using the context, and tracking can be set up and managed entirely
from the LLVMContext creation site (by adding the context to an
'automatically managed' set), rather than intruding to every Module
creation point.

We would still need to add a mutex to LLVMContext to make this thread safe
though.

-- Lang.

On Sun, Sep 16, 2018 at 10:22 AM David Blaikie <dblaikie at gmail.com> wrote:

> Agreed, the existing ownership seems sub-optimal. I wouldn't say broken,
> but subtle at least - looks like you get the choice to either manage the
> ownership of the Module object yourself, or let the context handle it (eg:
> currently it'd be valid to just do "{ LLVMContext C; new Module(C); new
> Module(C); }" - Modules end up owned by the context and cleaned up there).
>
> Might be hard to migrate existing users away from this without silently
> introducing memory leaks... maybe with some significant API breakage - move
> Module construction to a factory/helper that returns a
> std::unique_ptr<Module> - requiring every Module construction to be
> revisited, and users relying on LLVMContext based ownership/cleanup to
> redesign their code.
>
> As to the original question - gut reaction: this doesn't seem like
> something that's general-purpose enough to be implemented in the
> LLVMContext/Module itself. I think a reasonable ownership model for
> LLVMContext/Module is that the user is required to ensure the LLVMContext
> outlives all Modules created within it (same way a user of std::vector is
> required to ensure that the vector is not reallocated so long as they're
> keeping pointers/references to elements in it). I'd think/suggest
> ref-counted LLVMContext ownership would be done by wrapping/external
> tracking in this use case.
>
> - Dave
>
> On Sat, Sep 15, 2018 at 9:30 PM Lang Hames via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Actually, looking at the destructors for LLVMContext and Module I do not
>> think the current ownership scheme makes sense, so this might be a good
>> opportunity to re-think it.
>>
>> Right now an LLVMContext owns a list of modules (see
>> LLVMContextImpl::OwnedModules) that it destroys when its destructor is
>> called.  Modules remove themselves from this list if they are destructed
>> before the context:
>>
>> Module::~Module() {
>>   Context.removeModule(this);
>>   ...
>>
>> LLVMContextImpl::~LLVMContextImpl() {
>>   // NOTE: We need to delete the contents of OwnedModules, but Module's
>> dtor
>>   // will call LLVMContextImpl::removeModule, thus invalidating iterators
>> into
>>   // the container. Avoid iterators during this operation:
>>   while (!OwnedModules.empty())
>>     delete *OwnedModules.begin();
>>     ...
>>
>> This makes it unsafe to hold a unique_ptr to a Module: If any Module is
>> still alive when its context goes out of scope it will be double freed,
>> first by the LLVMContextImpl destructor and then again by the unique ptr.
>> Idiomatic scoping means that we tend not to see this in practice  (Module
>> takes an LLVMContext reference, meaning we always declare the context
>> first, so it goes out of scope last), but makes the context ownership
>> redundant: the modules are always freed first via their unique_ptr's.
>>
>> I don't think it makes sense for LLVMContext to own Modules. I think that
>> Modules should share ownership of their LLVMContext via a shared_ptr.
>>
>> Thoughts?
>>
>> Cheers,
>> Lang.
>>
>> On Sat, Sep 15, 2018 at 4:14 PM Lang Hames <lhames at gmail.com> wrote:
>>
>>> Hi All,
>>>
>>> ORC's new concurrent compilation model generates some interesting
>>> lifetime and thread safety questions around LLVMContext: We need multiple
>>> LLVMContexts (one per module in the simplest case, but at least one per
>>> thread), and the lifetime of each context depends on the execution path of
>>> the JIT'd code. We would like to deallocate contexts once all modules
>>> associated with them have been compiled, but there is no safe or easy way
>>> to check that condition at the moment as LLVMContext does not expose how
>>> many modules are associated with it.
>>>
>>> One way to fix this would be to add a mutex to LLVMContext, and expose
>>> this and the module count. Then in the IR-compiling layer of the JIT we
>>> could have something like:
>>>
>>> // Compile finished, time to deallocate the module.
>>> // Explicit deletes used for clarity, we would use unique_ptrs in
>>> practice.
>>> auto &Ctx = Mod->getContext();
>>> delete Mod;
>>> std::lock_guard<std::mutex> Lock(Ctx->getMutex());
>>> if (Ctx.getNumModules())
>>>   delete Ctx;
>>>
>>> Another option would be to invert the ownership model and say that each
>>> Module shares ownership of its LLVMContext. That way LLVMContexts would be
>>> automatically deallocated when the last module using them is destructed
>>> (providing no other shared_ptrs to the context are held elsewhere).
>>>
>>> There are other possible approaches (e.g. side tables for the mutex and
>>> module count) but before I spent too much time on it I wanted to see
>>> whether anyone else has encountered these issues or has opinions on
>>> solutions.
>>>
>>> Cheers,
>>> Lang.
>>>
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>> llvm-dev at lists.llvm.org
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>>
>
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