Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

Matthew Wilcox
2009-01-07T21:09:23Z

On Wed, Jan 07, 2009 at 12:55:49PM -0800, Linus Torvalds wrote:
> void loop_while_oncpu(struct mutex *lock, struct thread_struct *thread)
> {
> for (;;) {
> unsigned cpu;
> struct runqueue *rq;
>
> if (lock->owner != thread)
> break;
>
> /*
> * Need to access the cpu field knowing that
> * DEBUG_PAGEALLOC could have unmapped it if
> * the mutex owner just released it and exited.
> */
> if (__get_user(cpu, &thread->cpu))
> break;

I appreciate this is sample code, but using __get_user() on
non-userspace pointers messes up architectures which have separate
user/kernel spaces (eg the old 4G/4G split for x86-32). Do we have an
appropriate function for kernel space pointers? Is this a good reason
to add one?

--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."