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From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2007-07-19 01:36:32


Wilbur Lang wrote:
> Hi,
> As win32 version of shared mutexes are based "spin and wait", which
> leads to poor performance and priority inversions*.
> I have an idea about simulate PTHREAD_PROCESS_SHARED with
> DuplicateHandle() under win32.
> 1. anonymous mutex keeps the process id (e.g. hprocess) where
> CreateMutex called and HANDLE of mutex (e.g. hmutex).
> 2. if another process want to shared this mutex, it call
> DuplicateHandle() with hSourceProcessHandle=hprocess and
> hSourceHandle=hmutex, and return hmutex_dup.
> 3. the process use hmutex_dup to lock/unlock/try_lock/timed_lock
> 4. we can optimize performance with cache hmutex_dup.
> and a specialized scoped_lock required for anonymous mutex under
> windows which didn't use handle directly, but DuplicateHandle.

The main problem with storing the process id is that I want anonymous
mutexes to work with memory mapped files. If a process creates a managed
mapped file(which includes a mutex) fills it with data and then unmaps
it, it should be able to map it again and continue working. If several
processes map it and then unmap it, all references to that mutex are
destroyed and Windows would destroy the mutex. We'll need to re-create
the windows mutex although the object is still there in shared memory.

This behavior is possible with POSIX mutexes with PTHREAD_PROCESS_SHARED
attribute and it's the most difficult one to emulate in Windows.



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