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Date: Sat, 5 Sep 2015 08:18:36 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Carsten Emde <C.Emde@osadl.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        John Kacur <jkacur@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Clark Williams <clark.williams@gmail.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [RFC][PATCH RT 0/3] RT: Fix trylock deadlock without msleep()
 hack
Message-ID: <20150905081836.2967d80d@gandalf.local.home>
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References: <20150904011900.730816481@goodmis.org>
	<alpine.DEB.2.11.1509051217330.15006@nanos>
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On Sat, 5 Sep 2015 12:30:59 +0200 (CEST)
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. 

That was my original solution, and I believe I added patches to do
exactly that to the networking code in the past. I remember writing
that helper function such that on non PREEMPT_RT it was a nop.

I even had that solution in my slides at LinuxCon/LinuxPlumbers ;-)


But then I talk about dcache.c. Take a look at that file, and the
complexity of that. Is it safe to take the inode and dcache parent
locks after you unlock the other locks?

-- Steve