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Date: Sat, 5 Sep 2015 14:04:57 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Carsten Emde <C.Emde@osadl.org>,
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        Peter Zijlstra <peterz@infradead.org>,
        Clark Williams <clark.williams@gmail.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>
Subject: Re: [RFC][PATCH RT 0/3] RT: Fix trylock deadlock without msleep()
 hack
Message-ID: <20150905120457.GA21338@gmail.com>
References: <20150904011900.730816481@goodmis.org>
 <alpine.DEB.2.11.1509051217330.15006@nanos>
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* Thomas Gleixner <tglx@linutronix.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