linux-kernel - Re: Regression: WARNINGS and lockdep spews in 2.6.38-rc3+ (bisected).

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Message-ID: <1297184127.13327.142.camel@laptop>
Date:	Tue, 08 Feb 2011 17:55:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	Nick Bowler <nbowler@...iptictech.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Regression: WARNINGS and lockdep spews in 2.6.38-rc3+
 (bisected).

On Thu, 2011-02-03 at 19:42 +0800, Yong Zhang wrote:
> My patch is inspired by this http://lkml.org/lkml/2010/8/16/291

Right, so that splat warns about:

spin_lock_irqsave(&shost->host_lock);
del_timer_sync()
  spin_lock_irqsave(&t->split_timeout_timer);

vs

<SOFTIRQ>
  call_timer_fn()
    spin_lock(&t->split_timeout_timer);
    <IRQ>
      spin_lock_irqsave(&shost->host_lock);

Which Steven just pointed out is an actual deadlock because:


CPU0					CPU1
spin_lock_irqsave(&shost->host_lock);
					<SOFTIRQ>
					  call_timer_fn();
					    base->running_timer = timer;
					    func();
					      <IRQ>
						spin_lock(&shost->host_lock);
del_timer_sync()
  while(base->running_timer == timer);

And the world stops turning..


Also note that your patch changes the above to:

spin_lock_irqsave(&shost->host_lock);
del_timer_sync()
  spin_lock_bh(&t->split_timeout_timer);

Which wouldn't make one bit of difference.

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