Re: [PATCH][RFC]: mutex: adaptive spin

Matthew Wilcox
2009-01-06T23:09:59Z

On Tue, Jan 06, 2009 at 03:00:47PM -0800, Linus Torvalds wrote:
> Well, if you didn't go to sleep, a few more questions..

I know this one! Me sir, me me me!

> > int __sched
> > mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
> > {
> > + int ret;
> > +
> > might_sleep();
> > - return __mutex_lock_common(lock, TASK_KILLABLE, subclass, _RET_IP_);
> > + ret = __mutex_lock_common(lock, TASK_KILLABLE, subclass, _RET_IP_);
> > + if (!ret)
> > + lock->owner = current;
> > +
> > + return ret;
>
> This looks ugly. Why doesn't __mutex_lock_common() just set the lock
> owner? Hate seeing it done in the caller that has to re-compute common
> (yeah, yeah, it's cheap) and just looks ugly.

Because __mutex_lock_common() is the slow path. The fast path is a
couple of assembly instructions in asm/mutex.h. If the lock isn't
contended, it will never call __mutex_lock_common().

That would make the whole exercise rather pointless; the only time worth
spinning really is if you're the only other one waiting for it ... if
there's already a waiter, you might as well go to sleep.

--
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."