Re: [PATCH] rtmutex: multiple candidate owners without unrelated boosting

Thomas Gleixner
2010-12-15T14:16:54Z

On Wed, 15 Dec 2010, Steven Rostedt wrote:

> On Wed, 2010-12-15 at 09:02 +0100, Thomas Gleixner wrote:
> > On Tue, 14 Dec 2010, Steven Rostedt wrote:
> >
> > > On Tue, 2010-12-14 at 17:04 +0800, Lai Jiangshan wrote:
> > >
> > > OK, I was looking at this in a bit more detail (the coffee finally set
> > > in) and I was at first looking to nuke the cand_owner since it is
> > > redundant to cand_seq. But I think we can nuke the cand_seq instead and
> > > use the top_waiter as the decider.
> >
> > So you just use cand_owner (the name sucks) to flag that the waiter
> > has been woken up either by the boost code or by an unlock. The waiter
> > clears that flag with waiter->lock->wait_lock held before calling
> > schedule().
> >
> > Though I think we do need it at all. wakeup of an already running task
> > is almost a nop, so having one less state to worry about is good.
>
> I was hoping to remove it completely, and yes I was hoping we could
> because a wakeup of a woken task is almost a nop. But then I saw this in
> rt_mutex_adjust_prio_chain():
>
>
> > /*
> > * Check the orig_waiter state. After we dropped the locks,
> > * the previous owner of the lock might have released the lock
> > - * and made us the pending owner:
> > + * and made us candidate owner:
> > */
> > - if (orig_waiter && !orig_waiter->task)
> > + if (orig_waiter && orig_waiter->cand_owner)
> > goto out_unlock_pi;
> >
>
> I'm not sure what else we could use to check if the original waiter has
> been given the lock.

That does not matter. The interesting part is whether the lock on
which orig_waiter is blocked on was unlocked. Lai's follow up patch does:

+ if (orig_waiter && !rt_mutex_owner(orig_lock))
goto out_unlock_pi;

Thanks,

tglx