linux-kernel - Re: [PATCH RT] rt: Make cpu_chill() use hrtimer instead of msleep()

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Message-ID: <20140205115700.3107d0e2@gandalf.local.home>
Date:	Wed, 5 Feb 2014 11:57:00 -0500
From:	Steven Rostedt <rostedt@...dmis.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>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	John Kacur <jkacur@...hat.com>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	stable-rt@...r.kernel.org
Subject: Re: [PATCH RT] rt: Make cpu_chill() use hrtimer instead of msleep()

On Wed, 5 Feb 2014 11:51:25 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> 
> Ulrich Obergfell pointed out that cpu_chill() calls msleep() which is woken
> up by the ksoftirqd running the TIMER softirq. But as the cpu_chill() is
> called from softirq context, it may block the ksoftirqd() from running, in
> which case, it may never wake up the msleep() causing the deadlock.
> 
> I checked the vmcore, and irq/74-qla2xxx is stuck in the msleep() call,
> running on CPU 8. The one ksoftirqd that is stuck, happens to be the one that
> runs on CPU 8, and it is blocked on a lock held by irq/74-qla2xxx. As that
> ksoftirqd is the one that will wake up irq/74-qla2xxx, and it happens to be
> blocked on a lock that irq/74-qla2xxx holds, we have our deadlock.
> 
> The solution is not to convert the cpu_chill() back to a cpu_relax() as that
> will re-create a possible live lock that the cpu_chill() fixed earlier, and may
> also leave this bug open on other softirqs. The fix is to remove the
> dependency on ksoftirqd from cpu_chill(). That is, instead of calling
> msleep() that requires ksoftirqd to wake it up, use the
> hrtimer_nanosleep() code that does the wakeup from hard irq context.
> 
> Found-by: Ulrich Obergfell <uobergfe@...hat.com>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> 

I should have added:

Cc: stable-rt@...r.kernel.org

-- Steve
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