[llvm-dev] LLVMContext: Threads and Ownership.

[David Blaikie via llvm-dev]

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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