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

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Date:	Wed, 15 Dec 2010 15:16:28 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Lai Jiangshan <laijs@...fujitsu.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <hidave.darkstar@...il.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Namhyung Kim <namhyung@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] rtmutex: multiple candidate owners without unrelated
 boosting

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
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