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

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Date:	Mon, 14 Sep 2015 11:50:08 +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:

> > 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.
> 
> In the case of dcache::dentry_kill() we probably do not have to take refcounts 
> and it might be actually counterproductive to do so. y->x, i.e. dentry->parent, 
> cannot vanish under us, if I understand the life time rules correctly.

Ok, that's even better.

> Aside of that, yes, I was thinking about a similar scheme for that. I need some 
> more time to grok all the rules there :)

Ok, great! :-)

I really don't think we need a new locking primitive - and with something like the 
above we could improve the code upstream as well and make it scale better in some 
scenarios, right?

Thanks,

	Ingo
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