On Wed, 12 Jun 2002, Linus Torvalds wrote:
> Any amount of tracking would be _extremely_ expensive. Right now getting
> an uncontended lock is about 15 CPU cycles in user space.
>
> Tryin to tell the kernel about gettign that lock takes about 1us on a P4
> (system call overhead), ie we're talking 18000 cycles. 18 THOUSAND cycles
> minimum. Compared to the current 15 cycles. That's more than three orders
> of magnitude slower than the current code, and you just lost the whole
> point of doing this all in user space in the first place.
That doesn't rule out approaches like storing a cookie alongside the lock
once it's acquired (or in a parallel space). Which can easily be done with
a wrapper around lock acquisition. And stale lock detection needn't be
done in kernel space either.
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