On Thu, 2006-07-13 at 00:07 +0100, Alan Cox wrote:
> Ar Mer, 2006-07-12 am 16:26 -0600, ysgrifennodd Eric W. Biederman:
> > If the lock is not short lived then the release is like to be a long
> > ways off. If the lock is not highly contended then you are not likely
> > to hit the window when someone else as the contended lock.
> >
> > How frequent are highly contended short lived locks in user space?
>
> I'm not sure it matters.
>
> If you want to do the job right then do this
>
> - Stick an indicator of how much else wants to run on this CPU in the
> vsyscall page or similar location
Except that "this cpu" doesn't really mean anything in userspace, and
while I think Andi has some tricks to get some sort of CPU number to
userspace (though it's really only valid during the execution of the
instruction that reads it :) I haven't yet found an equivalent for
powerpc (and possibly other architectures will have the same problem).
> In your locks you can now do
>
> while(try_and_grab_lock() == FAILED) {
> if (kernelpage->waiting > 0)
> sys_somelockwaitthing()
> }
>
> Furthermore the kernel can be intelligent about the waiting indicator
> for power or other global scheduling reasons
>
> [Disclaimer: There is a patent issue around this technique but its not
> one that will impact GPL code as permissions are given for GPL use.]
>
> Alan
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