Speeding up wineserver synchronization objects with shared memory

[Robert O'Callahan]

Alexandre Julliard wrote:
> For a mutex the basic definition is obviously that you can never have
> two threads holding the mutex at the same time, and that when it is
> released some other thread has a chance at grabbing it. This has to be
> true no matter what the process does, including corrupting memory or
> crashing.

... but not including "process randomly decides to call AcquireMutex
again", presumably.

So your concern is simply that if it's too simple to lock the mutex, then
you have an unacceptably high probability of a runaway process
accidentally locking the mutex, whereas you regard the probability of the
process accidentally calling AcquireMutex again (or executing the
protected control transfer on its own) as acceptably low. That seems
reasonable.

You can vastly reduce the probability of accidental locking by changing
the client_locked field to an int and requiring a specific magic number to
be stored there to signal a taken lock. You can vary the magic number per
mutex, and store it in your auxiliary per-mutex data structures XORed with
some known constant, to ensure that the magic number is never stored
anywhere in memory except for the lock field.

But if "executes a protected control transfer" is your gold standard of an
action that is so unlikely to happen by chance it must be the intended
behaviour of the process, then of course no other solution will be
acceptable.

> You also need to make sure nothing a process does should be
> able to crash/hang the wineserver or another process (except if they
> fight over the same mutex of course),

I think we're there.

> and then there is the small
> matter of respecting the Win32 semantics...

Yeah, but that doesn't seem fundamental --- the wineserver can always take
ownership of the mutex itself when it needs to do a complex atomic
operation. I'll let Gavriel work out the details :-).

Rob
-- 
[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]