Re: [Boost-users] [Interprocess] hang locking p_hdr->m_mutex

Subject: Re: [Boost-users] [Interprocess] hang locking p_hdr->m_mutex
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2011-08-26 00:56:43

El 25/08/2011 20:53, David Byron escribió:

> Instead of using a persistent integer on windows, how about CreateMutex
> Then if the mutex holder dies, the other side learns about it because
> WaitForSingleObject returns WAIT_ABANDONED. But then it's not so
> important to learn that the other side is dead...just that we've got the
> mutex and it's OK to proceed.

The key problem is object lifetime. In windows, when last attached 
process dies/closes the handle, the interprocess mechanism is 
automatically destroyed. In Unix, it's more like a file and an explicit 
unlink must be done. That's the real problem of portable semantics 
between windows and unix.

> My understanding is that a process waiting on a posix interprocess_mutex
> wakes up (having taken ownership) if the owner process dies. Is that
> right? Adding an implementation for windows that behaves that way feels
> like a big bonus -- way better than hanging.

No. There is an option, robust mutexes, not implemented by all systems, 
that establishes a protocol for handling abandoned mutexes:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_getrobust.html

>> The queue is removed (in windows, the name of the queue is changed so
>> that no other connection succeeds and marked to be erased when the last
>> handle is closed,
>
> That's useful information, but what I'm also curious about is whether
> the process waiting for the mutex wakes up or not. Using the emulated
> interprocess_mutex, I'm not sure. Because the waiting process still
> likely has a handle, I guess it hangs forever, yes?

It will hang, yes, just like in a posix system without robust mutex 
support. Adding robust mutex support is in my to-do list, exploring some 
emulation for windows too, but I guess that would require a huge amount, 
because I don't know any portable runtime that has achieved this.

Best,

Ion