[llvm-dev] Issue with initialization order for ManagedStaticMutex

[Justin Holewinski via llvm-dev]

 We're seeing an issue with ManagedStaticMutex. There is a race condition
between mutex_init_flag and the call_once usage in getManagedStaticMutex,
at least on MSVC 2017.

The sequence of operations we see during startup are as follows:

1. The first global ManagedStatic initializes, causing
getManagedStaticMutex to call initializeMutex through std::call_once
2. Some N number of additional global ManagedStatic initialize, where
getManagedStaticMutex skips initializeMutex since it has already been called
3. The global constructor for "mutex_init_flag" runs, resetting the flag
indicating the call has already been made
4. Another global ManagedStatic initializes, causing getManagedStaticMutex
to be called, but this time initializeMutex is run since the
mutex_init_flag flag was reset

After (4), the original ManagedStaticMutex is leaked. Additionally, (1) and
(2) are operating on an uninitialized global variable.

As a fix, is there anything wrong with making mutex_init_flag
function-local in getManagedStaticMutex? Its only use is in that function,
and this would guarantee mutex_init_flag is initialized before use.


-- 

Thanks,

Justin Holewinski
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