Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

Chris Mason
2009-01-08T18:27:14Z

On Thu, 2009-01-08 at 10:16 -0800, Linus Torvalds wrote:
>
> On Thu, 8 Jan 2009, Steven Rostedt wrote:
> >
> > Ouch! I think you are on to something:
>
> Yeah, there's somethign there, but looking at Chris' backtrace, there's
> nothing there to disable preemption. So if it was this simple case, it
> should still have preempted him to let the other process run and finish
> up.
>

My .config has no lockdep or schedule debugging and voluntary preempt.
I do have CONFIG_INLINE_OPTIMIZE on, its a good name for trusting gcc I
guess.

> So I don't think Chris' softlockup is at least _exactly_ that case.
> There's something else going on too.
>
> That said, I do think it's a mistake for us to care about the value of
> "spin_on_owner()". I suspect v8 should
>
> - always have
>
> if (need_resched())
> break
>
> in the outer loop.
>
> - drop the return value from "spin_on_owner()", and just break out if
> anything changes (including the need_resched() flag).
>
> - I'd also drop the "old_value < 0 &&" test, and just test the
> list_empty() unconditionally.
>

I'll give the above a shot, and we can address the preempt + !owner in
another rev

-chris