From mboxrd@z Thu Jan  1 00:00:00 1970
From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: Observed deadlock in ext4 under 3.2.23-rt37 & 3.2.33-rt50
Date: Thu, 3 Jan 2013 09:18:37 -0500
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Cc: Staffan Tjernstrom <stjernstrom@eagleseven.com>,
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To: Steven Rostedt <rostedt@goodmis.org>
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On Thu, Jan 03, 2013 at 08:21:31AM -0500, Steven Rostedt wrote:
> Anytime you have something that does the following in order to break
> lock ordering:
> 
> repeat:
> 	lock(A);
> 	<do something>
> 	if (!trylock(B)) {
> 		unlock(A);
> 		cpu_relax();
> 		goto repeat;
> 	}
> 
> 
> We can live lock, because spinlocks in -rt turn into a mutex. Thus, the
> holder of lock B may not be on another CPU but actually on the current
> CPU and is waiting for the process that is in this loop. If that process
> happens to be an RT task, then the system stops.

Good point, but I'm prety sure we don't have any kind of trylock loops
in either fs/ext4 or fs/jbd2, though.

					- Ted