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From: Greg Colvin (greg_at_[hidden])
Date: 2000-08-24 23:04:16

From: William Kempf <sirwillard_at_[hidden]>
> --- In boost_at_[hidden], "Greg Colvin" <greg_at_c...> wrote:
> > From: William Kempf <sirwillard_at_m...>
> > > --- In boost_at_[hidden], "Greg Colvin" <greg_at_c...> wrote:
> > > > From: William Kempf <sirwillard_at_m...>
> > > In corner cases this may not be true. For instance, trying to
> > > implement a Win32 Event using a monitor the implementation of Signal()
> > > could be greatly optimized by using InterlockedIncrement to change
> > > the "boolean" flag to true and calling the CVs notify_all() outside
> > > of a monitor lock. This is just the simplest example I can think
> > > of... there may be more corner cases. Not a great reason to require
> > > this, I know. But the fact that it's going to be problematic to do
> > > any more than call it "undefined behavior" to call notify() outside
> > > of a monitor lock is impetus enough for me to at least attempt to
> > > define it as safe.
> >
> > This is another reason I like my idea of wrapping up the lock,
> > wait, and notify in a single constructor. It means that the
> > user can't get into undefined behavior, and the implementer
> > has maximum freedom to optimize.
> However, this puts restrictions on the user. This results in the
> optimal code above not being an option.

I think it is indeed an option: in the constructor you aquire the
lock and wait, in the destructor you release the lock and notify.
The choice of primitives and order of operations is up to you. So
if you want to use an interlocked increment outside the mutex you
can, with confidence that the user won't slip another mutex in
between. Also up to you is whether to use a while..wait in the
constructor and notify_all in the destructor, or to use an if..wait
in the constructor and a notify_one in the destructor.

> It also means that there's
> only two places within the monitor that you can wait, the beginning
> and the end. I can see design patterns where you'd wait in the
> middle.
> If there's truly a danger for undefined behavior I might be persuaded
> to agree to a wait_and_lock object though it still bothers me that a
> notify will be part of this scheme. It doesn't seem natural or
> expressive enough.

I'll do some more research, but in the examples I have seen the wait
is always at the beginning and the notify is always at the end.
Anyway, this object needn't be the *only* way to wait and notify.

I'm reviewing our (well documented!) graph library, and wondering
more and more if we can't make some use of it here. It seems that
a well designed monitor amounts to a cyclic graph with conditions at
the nodes and the code that waits on one condition and notifies
others as the edges. It seems further that being deadlock-free is a
deducible property of the graph.

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