Speeding up wineserver synchronization objects with shared memory

Robert O'Callahan roc+ at cs.cmu.edu
Sat Feb 24 16:16:56 CST 2001

Ove Kaaven wrote:
> On Thu, 22 Feb 2001, Robert O'Callahan wrote:
> > Then on some other thread in the same address space
> > that serves wineserver requests:
> There's no such thread, and will never be such a thread (having the
> wineserver calling into client threads is an inherently unstable
> design).

The wineserver need not depend on the client thread responding to its
(asynchronous) messages. In this case, if the client ignores the
messages,  you get the same effect as if it never releases its mutex,
which is not a new failure mode.

Actually there is another design that doesn't need a per-client thread.
Instead it uses a per-client memory block shared between the client and
the wineserver. This memory block holds the lock associated with each
mutex, and the wineserver itself does the actions that I previously
ascribed to the client thread. (It's a bit tricky because the wineserver
never wants to block on acquiring the lock in shared memory; you have to
set some flag in the lock word, wait for the client process to send you a
message that it's released its mutex lock, and then try again.) As long as
the wineserver maintains its own record of who owns each mutex, it need
not actually trust any of the contents of the shared memory blocks. Other
than not requiring separate client threads, this design also requires
fewer process context switches. In the non-contended case, acquiring a
mutex previously held by another process would require only a switch to
the wineserver and back.

Sorry I'm not familiar with the Wine code and I don't have time to dig
into it, but I just want to urge you not to give up on finding a safe,
very efficient solution.

[Robert O'Callahan http://www.cs.cmu.edu/~roc 7th year CMU CS PhD student
"Now when Joshua was near Jericho, he looked up and saw a man standing in
front of him with a drawn sword in his hand. Joshua went up to him and
asked, 'Are you for us or for our enemies?' 'Neither,' he replied, 'but
as commander of the army of the LORD I have now come.'" - Joshua 5:13-14]

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