Date: Thu, 30 Apr 2015 15:07:24 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Austin Schuh <austin@peloton-tech.com>
Cc: Peter Zijlstra <peterz@infradead.org>, LKML <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Clark Williams <williams@redhat.com>,
        Dave Chinner <david@fromorbit.com>
Subject: Re: [PATCH][RT] xfs: Disable preemption when grabbing all icsb
 counter locks
Message-ID: <20150430150724.519d4054@gandalf.local.home>
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	<20150430143226.061202df@gandalf.local.home>
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On Thu, 30 Apr 2015 11:40:07 -0700
Austin Schuh <austin@peloton-tech.com> wrote:

> On Thu, Apr 30, 2015 at 11:32 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> > On Thu, 30 Apr 2015 20:07:21 +0200
> > Peter Zijlstra <peterz@infradead.org> wrote:
> >> The irony, this is distinctly non deterministic code you're putting
> >> under a RT specific preempt_disable ;-)
> >
> > I know :-(
> >
> > Unfortunately, a RT behaving fix would be much more invasive and would
> > probably require the help of the xfs folks. For now, this just prevents
> > a live lock that can happen and halt the system, where it becomes
> > deterministic catastrophe.
> >
> > -- Steve
> 
> Would it work to instead create a lock to replace the
> preempt_enable_rt/preempt_disable_rt pair in XFS?

Not just the place where the preempt_enable and disable is done, but it
would need to replace all the per cpu bit spin locks (the
XFS_ICSB_FLAG_LOCK bit in icsbp->icsb_flags).

If we can replace them with a rtmutex (spin_lock() in vanilla kernel),
then that would work.

-- Steve

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