[LLVMdev] PassRegistry thread safety and ManagedStatic interaction

[David Blaikie]

On Mon, Jun 2, 2014 at 10:00 AM, Zachary Turner <zturner at google.com> wrote:
> I accidentally a word.   I meant to say the goal would be to enforce
> once-only *initialization* inside of ManagedStatic.
>
>
> On Mon, Jun 2, 2014 at 9:59 AM, Zachary Turner <zturner at google.com> wrote:
>>
>> The mutex could be made an actual global static instead of a
>> ManagedStatic.  This guarantees it would be constructed before main, and
>> live until the end of main.

Except being live before main is insufficient. Passes are registered
from other global initializers. There's no ordering guarantee between
global initializers, so it's possible that pass registrations may
occur before the global mutex is initialized... (at least I think
that's an issue?)

>> So even in PassRegistry's destructor, which
>> would get call during llvm_shutdown(), it would always have the same mutex.
>> Ideally I'd like to just delete the mutex, as it doesn't seem like anyone is
>> using it in a multi-threaded fashion, but I could be wrong about that, so
>> I'll wait for more information.
>>
>> Eventually the goal would be to actually enforce once-only inside of
>> ManagedStatic (I found this bug actually by changing ManagedStatic to sue
>> std::call_once), so we could prevent problems like this from ever happening
>> in the future.

Agreed. I've seen other nasty bugs with insufficiently constrained
singletons that allow reconstruction in some cases... I'd love to have
ManagedStatic be more restrictive about how it can be used.

One other wrinkle is that, at least in theory, LLVM can be used after
llvm_shutdown (llvm_shutdown just brings everything back to the
startup state... so then it can all run again)... so that constraint
can't be added to ManagedStatic. They're meant to reinitialize on
first use after llvm_shutdown :/

>>
>>
>>
>>
>> On Mon, Jun 2, 2014 at 9:53 AM, David Blaikie <dblaikie at gmail.com> wrote:
>>>
>>> On Mon, Jun 2, 2014 at 9:39 AM, Zachary Turner <zturner at google.com>
>>> wrote:
>>> > I actually had an idea about how to fix this in a relatively painless
>>> > manner.  Although given my experience over the past 4 days, it might
>>> > not be
>>> > best to call it painless without first trying :)
>>> >
>>> > The idea is to make a StaticPassRegistry.  RegisterPass<> only touches
>>> > the
>>> > StaticPassRegistry, and nothing else touches the StaticPassRegistry.
>>> > So
>>> > once you enter main(), StaticPassRegistry can be considered immutable.
>>> > In
>>> > main(), the existing PassRegistry initializes itself from the
>>> > StaticPassRegistry.  This *should* solve all the problems, the only
>>> > trick is
>>> > finding every executable that uses the PassRegistry.
>>>
>>> How does this address the llvm_shutdown/pass derigstration issue?
>>>
>>> (a little more of my tale: I started trying to remove PassRegistry's
>>> pimpl, but noticed that during pass derigstration the PassRegistry had
>>> already been destroyed (since it's created by the first global that
>>> tries to register a pass, so it must be destroyed before that global
>>> in turn tries to deregister the pass) and what would happen is that
>>> the global would be reconstructed, its pimpl would be reinitialized to
>>> null and the deregistration function would stop there... subtle and
>>> annoying)
>>>
>>> >
>>> > it's times like this I wish we have an LLVMInitialize() function which
>>> > every
>>> > executable using LLVM is required to call early in main().
>>> >
>>> >
>>> > On Mon, Jun 2, 2014 at 9:14 AM, David Blaikie <dblaikie at gmail.com>
>>> > wrote:
>>> >>
>>> >> Yeah, I ran into this a few weeks ago trying to tidy up ownership of
>>> >> the PassRegistry and it made me sad. Chatting to Chandler he seemed to
>>> >> be of the opinion that the whole thing was a rats nest of bad & not
>>> >> worth my time (though perhaps it's worth yours, I'm not sure).
>>> >>
>>> >> Chandler - was this just going to "go away" in the the glorious new
>>> >> pass manager future?
>>> >>
>>> >> On Sun, Jun 1, 2014 at 10:36 AM, Zachary Turner <zturner at google.com>
>>> >> wrote:
>>> >> > +cc original authors of these changes.
>>> >> >
>>> >> > Is PassRegistry intended to be thread-safe?  The header file
>>> >> > explicitly
>>> >> > says
>>> >> > that PassRegistry is not thread-safe, but there are mutexes and
>>> >> > locking
>>> >> > used
>>> >> > in the code.  This is actually creating a problem, because of a
>>> >> > subtle
>>> >> > bug
>>> >> > involving static initialization order and shutdown.
>>> >> >
>>> >> > In particular, the RegisterPass<> static template will get invoked
>>> >> > during
>>> >> > static initialization and call
>>> >> >
>>> >> >   PassRegistry::getPassRegistry()->registerPass(*this);
>>> >> >
>>> >> > Note that PassRegistry, however, is a ManagedStatic.  So the call to
>>> >> > getPassRegistry() creates the backing object of the ManagedStatic
>>> >> > here.
>>> >> > Then registerPass gets called, which attempts to lock the mutex.
>>> >> > This
>>> >> > will
>>> >> > initialize the backing object of the SmartRWMutex.
>>> >> >
>>> >> > During shutdown, it happens in reverse order.  First the
>>> >> > SmartRWMutex is
>>> >> > destroyed, then the PassRegistry is destroyed.  During the
>>> >> > PassRegistry's
>>> >> > destructor, it attempts to lock the mutex again.  This works in the
>>> >> > current
>>> >> > code because ManagedStatic "accidentally" allocates another
>>> >> > SmartRWMutex.
>>> >> > However, the current implementation of ManagedStatic is already
>>> >> > buggy
>>> >> > for
>>> >> > other reasons, which I've tried to fix, and am now running into this
>>> >> > as
>>> >> > a
>>> >> > result of my fix  (true once-only initialization of ManagedStatics).
>>> >> >
>>> >> > I'm curious about the history here.  Can we remove the locking from
>>> >> > PassRegistry?  And what would it take to get RegisterPass<> to not
>>> >> > rely
>>> >> > on
>>> >> > static initialization.  Is there any reason why we can't just
>>> >> > initialize
>>> >> > these at runtime early in main?
>>> >> >
>>> >> > _______________________________________________
>>> >> > LLVM Developers mailing list
>>> >> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>> >> >
>>> >
>>> >
>>
>>
>