linux-kernel - Re: [RFC][PATCH RT 0/3] RT: Fix trylock deadlock without msleep() hack

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Date:	Tue, 8 Sep 2015 09:31:16 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Carsten Emde <C.Emde@...dl.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	John Kacur <jkacur@...hat.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clark.williams@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [RFC][PATCH RT 0/3] RT: Fix trylock deadlock without msleep()
 hack


* Thomas Gleixner <tglx@...utronix.de> wrote:

> 3) sched_yield() makes me shudder
> 
>    CPU0			CPU1	
> 
>    taskA
>      lock(x->lock)
> 
>    preemption
>    taskC
> 			taskB
> 			  lock(y->lock);
> 			  x = y->x;
> 			  if (!try_lock(x->lock)) {
> 			    unlock(y->lock);
> 			    boost(taskA);
> 			    sched_yield();  <- returns immediately

So I'm still struggling with properly parsing the usecase.

If y->x might become invalid the moment we drop y->lock, what makes the 'taskA' 
use (after we've dropped y->lock) safe? Shouldn't we at least also have a 
task_get(taskA)/task_put(taskA) reference count, to make sure the boosted task 
stays around?

And if we are into getting reference counts, why not solve it at a higher level 
and get a reference count to 'x' to make sure it's safe to use? Then we could do:

        lock(y->lock);
retry:
	x = y->x;
        if (!trylock(x->lock)) {
		get_ref(x->count)
                unlock(y->lock);
                lock(x->lock);
                lock(y->lock);
		put_ref(x->count);
		if (y->x != x) { /* Retry if 'x' got dropped meanwhile */
			unlock(x->lock);
			goto retry;
		}
        }

Or so.

Note how much safer this sequence is, and still just as fast in the common case 
(which I suppose is the main motivation within dcache.c?).

Thanks,

	Ingo
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