On Thu, 21 Jun 2007, Linus Torvalds wrote:
>
> We don't do nesting locking either, for exactly the same reason. Are
> nesting locks "easier"? Absolutely. They are also almost always a sign of
> a *bug*. So making spinlocks and/or mutexes nest by default is just a way
> to encourage bad programming!
Side note, and as a "truth in advertising" section: I'll have to admit
that I argued against fair semaphores on the same grounds. I was wrong
then (and eventually admitted it, and we obviously try to make our mutexes
and semaphores fair these days!), and maybe I'm wrong now.
If somebody can actually come up with a sequence where we have spinlock
starvation, and it's not about an example of bad locking, and nobody
really can come up with any other way to fix it, we may eventually have to
add the notion of "fair spinlocks".
So my arguments are purely pragmatic. It's not that I hate fairness per
se. I dislike it only when it's used to "solve" (aka hide) other problems.
In the end, some situations do need fairness, and the fact that aiming for
fairness is often harder, slower, and more complicated than not doing so
at that point turns into a non-argument. If you need it, you need it.
I just don't think we need it, and we're better off solving problems other
ways.
(For example, we might also solve such problems by creating a separate
"fair_spin_lock" abstraction, and only making the particular users that
need it actually use it. It would depend a bit on whether the cost of
implementing the fairness is noticeable enough for it to be worth having
a separate construct for it).
Linus
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