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

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Message-ID: <20150430180721.GX5029@twins.programming.kicks-ass.net>
Date:	Thu, 30 Apr 2015 20:07:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	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, Apr 30, 2015 at 12:33:03PM -0400, Steven Rostedt wrote:
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index 51435dbce9c4..dbaa1ce3f308 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -1660,6 +1660,12 @@ xfs_icsb_lock_all_counters(
>  	xfs_icsb_cnts_t *cntp;
>  	int		i;
>  
> +	/*
> +	 * In PREEMPT_RT, preemption is not disabled here, and it
> +	 * must be to take the xfs_icsb_lock_cntr.
> +	 */
> +	preempt_disable_rt();
> +
>  	for_each_online_cpu(i) {
>  		cntp = (xfs_icsb_cnts_t *)per_cpu_ptr(mp->m_sb_cnts, i);
>  		xfs_icsb_lock_cntr(cntp);
> @@ -1677,6 +1683,8 @@ xfs_icsb_unlock_all_counters(
>  		cntp = (xfs_icsb_cnts_t *)per_cpu_ptr(mp->m_sb_cnts, i);
>  		xfs_icsb_unlock_cntr(cntp);
>  	}
> +
> +	preempt_enable_rt();
>  }

The irony, this is distinctly non deterministic code you're putting
under a RT specific preempt_disable ;-)
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