Re: Detecting OS loader lock

While this is nice in theory, it might not be always applicable.

We just faced a problem with multiple threads and a loader lock. One thread
did end, releasing a DLL (not from us) as a side effect (which means getting
the loader lock). Inside this THREAD_DETACH, the DLL called
GetProcAddress(), which in turn did trigger a DLL of ours, which in turn
tried to aquired a CriticalSection internally (which protects a list of
modules). This is because we need own skinning and we need to catch
GetProcAddress() calls.

Not the other thread did aquire the CriticalSection and did try to get the
module's handle (GetModuleHandle(), innocent enough), which internally tries
to get the loader lock.

Duh. Deadlock.

Knowing the loader lock we'd know we max not call some system APIs.

We have a workaround for this special case, but this just works around this
special case.

And the morale of the story: while it's fine to say that the loader lock is
none of our business, and we should stay clear to any of these issues
internally, we cannot rely on other DLLs to behave the same way, so we (as
developers) should be able to check that lock to avoid the chances of
deadlocks due to interoperability! Especially not in multithreaded
environments.

If this were that easy, why was "TryEnterCriticalSection" ever programmed?
For the same reason it would be very nice (yet years too late!) to either
get the loader lock's state, or a "try" version of critical APIs like
GetModuleHandle().

Christian

""Jeffrey Tan[MSFT]"" <jetan@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:nZcZyrHDIHA.5712@xxxxxxxxxxxxxxxxxxxxxxxxx
Hi Christoph,

Internally, OS loader lock is maintained in the PEB. But, we should not
look at the loader lock value in the PEB, not even for debugging purposes,
and especially not in libraries which might be reused.

In general, making any assumptions about undocumented system state is a
recipe for app compat problems down the line.

The recommended way is using AppVerifier. AppVerifier has checks that make
sure DLLs aren't doing anything wrong in DllMain. I think you can also run
with these checks on when running from Visual Studio with AppVerifier.
There's also a whitepaper on DLL best practices at
http://download.microsoft.com/download/a/f/7/af7777e5-7dcd-4800-8a0a-b183365
65f5b/DLL_bestprac.doc.

Hope this helps.

Best regards,
Jeffrey Tan
Microsoft Online Community Support
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