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

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Date:	Sat, 5 Sep 2015 14:04:57 +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:

> So the problem we need to solve is:
> 
> retry:
> 	lock(B);
> 	if (!try_lock(A)) {
> 		unlock(B);
> 		cpu_relax();
> 		goto retry;
> 	}
> 
> So instead of doing that proposed magic boost, we can do something
> more straight forward:
> 
> retry:
> 	lock(B);
> 	if (!try_lock(A)) {
> 		lock_and_drop(A, B);
> 		unlock(A);
> 		goto retry;
> 	}
> 
> lock_and_drop() queues the task as a waiter on A, drops B and then
> does the PI adjustment on A. 
> 
> Thoughts?

So why not do:

	lock(B);
	if (!trylock(A)) {
		unlock(B);
		lock(A);
		lock(B);
	}

?

Or, if this can be done, why didn't we do:

	lock(A);
	lock(B);

to begin with?

i.e. I'm not sure the problem is properly specified.

Thanks,

	Ingo
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