Re: [boost] Re: read/write mutexes and boost

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2003-10-20 18:36:39

--- Alexander Terekhov <terekhov_at_[hidden]> wrote:

[...]

> Real mutexes are designed for memory sharing, not isolation.
> 
> "Some have complained that POSIX mutexes don't allow a locking thread 
>  to detect that the previous owner terminated "abnormally"... that's 
>  because it wouldn't do any good to know without isolation between 
>  the entities involved. With IPC mechanisms it makes sense to know, 
>  because you can do something about it."
> 
> "Counting semaphores are well matched to dealing with 
>  producer/consumer problems, including those that might exist 
>  between threads of different processes, or between a signal handler 
>  and a thread. In the former case, there may be LITTLE OR NO MEMORY 
>  shared by the processes; in the latter case, one is not 
>  communicating between co-equal threads, but between a thread and an 
>  interrupt-like entity. It is for these reasons that IEEE Std 
>  1003.1-2001 allows semaphores to be used by threads."

That is right and interestingly enough Win32 doesn't have
WAIT_ABANDONED state for semaphors or any other synch.
object but mutex.

Actually Win32 makes it even more confusing. 
In the WAIT_ABANDONED state, the mutex is set to nonsignaled
but the ownership is granted. Isn't it weird!

However this thread is about mutexes on Win32 and
under this OS, WAIT_ABANDONED is a valid state
that needs to be handled.

In the current boost::mutex implementation 
the return code is ignored (in non-debug version),
so that if the mutex owner is terminated,
a waiting thread will get the ownership as if
nothing happened. It is dead wrong. 
In general WAIT_ABANODONED is not a good thing,
the data could have been partially modified
and the application should take appropriate actions.
That's said, in certain cases (like read-only protection)
even if a mutex is abandoned, the application can 
still continue to work normally. 

BTW: What happens with POSIX mutexes when the owner is terminated?

Eugene

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