linux-kernel - Re: 3.4.4-rt13: btrfs + xfstests 006 = BOOM.. and a bonus rt_mutex deadlock report for absolutely free!

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Date:	Tue, 17 Jul 2012 00:27:58 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mike Galbraith <efault@....de>
Cc:	Chris Mason <chris.mason@...ionio.com>,
	"Chris L. Mason" <clmason@...ionio.com>,
	"linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: 3.4.4-rt13: btrfs + xfstests 006 = BOOM..  and a bonus rt_mutex
 deadlock report for absolutely free!

On Tue, 2012-07-17 at 06:18 +0200, Mike Galbraith wrote:
>  
> > There's that too. But the issue I was talking about is with all trylock
> > loops. As holding an rt-mutex now disables migration, if a high priority
> > process preempts a task that holds the lock, and then the high prio task
> > starts spinning waiting for that lock to release, the lower priority
> > process will never get to run to release it. The cpu_chill() doesn't
> > help.
> 
> Hrm.  I better go make a testcase, this one definitely wants pounding
> through thick skull.

Actually, I was mistaken. I forgot that we defined 'cpu_chill()' as
msleep(1) on RT, which would keep a deadlock from happening.

It doesn't explain the performance enhancement you get :-/

-- Steve


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