From mboxrd@z Thu Jan  1 00:00:00 1970
From: Chris Mason <chris.mason@oracle.com>
Subject: Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning
Date: Thu, 08 Jan 2009 14:17:53 -0500
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@elte.hu>, paulmck@linux.vnet.ibm.com,
	Gregory Haskins <ghaskins@novell.com>,
	Matthew Wilcox <matthew@wil.cx>,
	Andi Kleen <andi@firstfloor.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Nick Piggin <npiggin@suse.de>,
	Peter Morreale <pmorreale@novell.com>,
	Sven Dietrich <SDietrich@novell.com>
To: Steven Rostedt <rostedt@goodmis.org>
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On Thu, 2009-01-08 at 13:14 -0500, Steven Rostedt wrote:
> 
> 
> On Thu, 8 Jan 2009, Steven Rostedt wrote:
> > > In fact, you might not even need a process C: all you need is for B to be 
> > > on the same runqueue as A, and having enough load on the other CPU's that 
> > > A never gets migrated away. So "C" might be in user space.
> 
> You're right about not needing process C.
> 
> > > 
> > > I dunno. There are probably variations on the above.
> > 
> > Ouch! I think you are on to something:
> > 
> >         for (;;) {
> >                 struct thread_info *owner;
> > 
> >                 old_val = atomic_cmpxchg(&lock->count, 1, 0);
> >                 if (old_val == 1) {
> >                         lock_acquired(&lock->dep_map, ip);
> >                         mutex_set_owner(lock);
> >                         return 0;
> >                 }
> > 
> >                 if (old_val < 0 && !list_empty(&lock->wait_list))
> >                         break;
> > 
> >                 /* See who owns it, and spin on him if anybody */
> >                 owner = ACCESS_ONCE(lock->owner);
> > 
> > The owner was preempted before assigning lock->owner (as you stated).
> 
> If it was the current process that preempted the owner and these are RT 
> tasks pinned to the same CPU and the owner is of lower priority than the 
> spinner, we have a deadlock!
> 
> Hmm, I do not think the need_sched here will even fix that :-/

RT tasks could go directly to sleeping.  The spinner would see them on
the list and break out.

-chris