linux-kernel - Re: [PATCH][RT] xfs: Disable preemption when grabbing all icsb counter locks

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Date:	Thu, 30 Apr 2015 15:07:24 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Austin Schuh <austin@...oton-tech.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH][RT] xfs: Disable preemption when grabbing all icsb
 counter locks

On Thu, 30 Apr 2015 11:40:07 -0700
Austin Schuh <austin@...oton-tech.com> wrote:

> On Thu, Apr 30, 2015 at 11:32 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Thu, 30 Apr 2015 20:07:21 +0200
> > Peter Zijlstra <peterz@...radead.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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