Subj : Re: recursive mutexes
To   : comp.programming.threads
From : Uenal Mutlu
Date : Sat May 21 2005 12:15 am

"David Schwartz"  wrote
>
> "Uenal Mutlu"  wrote in message
>
> >>     As I said, assuming it's the last unlock.
>
> > Wake not necessary at all (at least in my implementations of both
> > recursive and non-recursive mutex methods).
>
>     You can always trade-off expenses in one place by moving them to
> someplace else. That's why I don't make the argument that recursive mutexes
> are bad because they're more expensive than non-recursive mutexes.
>
>     They are in fact more expensive in every implementation I've ever seen
> that didn't just deliberately impose the expense on non-recursive mutexes as
> well. You can't avoid the fact that an 'Unlock' has to determine whether or
> not it's actually going to release the mutex as an additional expense not
> present in a non-recursive mutex. You can only remove that expense by making
> 'Lock' much more expensive so that it can handle the case where it failed to
> get the lock but still blocked other threads.
>
>     If you think I'm wrong, prove it. Present an implementation of a
> recursive mutex such that it can't be made significantly less expensive by
> making it non-recursive. I'll even ignore the overhead of the extra
> locks/unlocks, I'm only concerned with the cost of the first lock and the
> first unlock. If you can do it, I will call you a genius.
>
>     That said, it really doesn't matter. The problem with recursive mutexes
> is not that they hurt performance, because that's generally negligible. It's
> that they hide serious design flaws and promote sloppy design. Their *only*
> use is to allow you to write code that manipulates an object without knowing
> whether or not it holds a lock on that object. And that is just bad, bad,
> bad.

IMO, this all has nothing to do with what you are replying to.

And besides this, we already had "cleared" all these things above by simply
finding not a consensus, rather each continued believing in his own view.
Ie. I prefer the intelligent use of recursive mutex to simplify and ease the development,
whereas most who participated in these discussions stated that they find the
use of recursive mutexes a bad idea and that they prefer to use non-recursive
mutex only (if that's only true for a really complex system...)
And, I can imagine people who develop for Windows and do use
CriticalSection that they probably aren't aware of the fact that they actually
are using a recursive mutex.

.