Re: [PATCH] compat: Allow static initializer for pthreads on Windows

> In Windows it is not possible to have a static initialized mutex as of
> now, but that seems to be painful for the upcoming refactoring of the
> attribute subsystem, as we have no good place to put the initialization
> of the attr global lock.
>
> The trick is to get a named mutex as CreateMutex[1] will return the
> existing named mutex if it exists in a thread safe way, or return
> a newly created mutex with that name.
>
> Inside the critical section of the single named mutex, we need to double
> check if the mutex was already initialized because the outer check is
> not sufficient.
> (e.g. 2 threads enter the first condition `(!a)` at the same time, but
> only one of them will acquire the named mutex first and proceeds to
> initialize the given mutex a. The second thread shall not re-initialize
> the given mutex `a`, which is why we have the inner condition on `(!a)`.
>
> Due to the use of memory barriers inside the critical section the mutex
> `a` gets updated to other threads, such that any further invocation
> will skip the initialization check code altogether on the first condition.
>
> [1] https://msdn.microsoft.com/en-us/library/windows/desktop/ms682411(v=vs.85).aspx
>
> Signed-off-by: Stefan Beller <sbeller@google.com>
> ---
>
>  Flying blind here, i.e. not compiled, not tested. For a system I do not
>  have deep knowledge of. The only help was the online documentation.