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Date: Sat, 5 Sep 2015 08:50:48 -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: <20150905085048.5cefbf5b@gandalf.local.home>
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On Sat, 5 Sep 2015 08:18:36 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Sat, 5 Sep 2015 12:30:59 +0200 (CEST)
> Thomas Gleixner <tglx@linutronix.de> wrote:
> > 
> > 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.

Just to point out again that I misread what you wrote. That's what I
get for responding to email 10 minutes after I get out of bed ;-)


You need to be careful about adding the waiter on A. If the owner of A
is blocked on B, the pi inheritance may detect that as a deadlock.

-- Steve