[LLVMdev] PassRegistry thread safety and ManagedStatic interaction

[David Blaikie]

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?
>> >
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>> >
>
>