linux-kernel - Re: [ANNOUNCE] 3.2.9-rt17

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Date:	Thu, 08 Mar 2012 22:54:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [ANNOUNCE] 3.2.9-rt17

On Thu, 2012-03-08 at 16:44 -0500, Steven Rostedt wrote:
> On Thu, 2012-03-08 at 22:37 +0100, Peter Zijlstra wrote:
> 
> > > Now when the original task releases the lock again, the other task can
> > > take it just like it does on mainline.
> > 
> > Now interleave it with a third task of even higher priority that puts
> > the spinner to sleep.
> 
> So? It will eventually have to allow the task to run. Adding a "third
> higher priority" task can cause problems in any other part of the -rt
> kernel.
> 
> We don't need to worry about priority inversion. If the higher task
> blocks on the original task, it will boost its priority (even if it does
> the adaptive spin) which will again boost the task that it preempted.
> 
> Now we may need to add a sched_yield() in the adaptive spin to let the
> other task run.

That's not what I mean,..

task-A (cpu0)	task-B (cpu1)		task-C (cpu1)

		lock ->d_lock
lock ->i_lock
lock ->d_lock
			<-------------- preempts B
		trylock ->i_lock


While is is perfectly normal, the result is that A stops spinning and
goes to sleep. Now B continues and loops ad infinitum because it keeps
getting ->d_lock before A because its cache hot on cpu1 and waking A
takes a while etc..

No progress guarantee -> fail.

Test-and-set spinlocks have unbounded latency and we've hit pure
starvation cases in mainline. In fact it was so bad mainline had to grow
ticket locks to cope -- we don't want to rely on anything like this in
RT.
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