linux-kernel - Re: [PATCH RFC tip/core/rcu 7/7] rcu: Quiet RCU-lockdep warnings involving interrupt disabling

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Date:	Tue, 06 Dec 2011 11:27:26 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org
Subject: Re: [PATCH RFC tip/core/rcu 7/7] rcu: Quiet RCU-lockdep warnings
 involving interrupt disabling

On Tue, 2011-12-06 at 10:52 +0100, Peter Zijlstra wrote:
> On Tue, 2011-12-06 at 09:26 +0800, Yong Zhang wrote:
> 
> > Yeah, because we call might_sleep() in rt_mutex_lock() unconditionally.
> > But in this case the 'BUG: sleeping function called from invalid context
> > at *' is obviously false positive.
> 
> Why can't this mutex acquisition not block?

Gaah!! I see, this 5342e269 patch is revolting.. guys that's really vile
don't do that!

I tried reading the RCU code but I gave up.. rcu_boost() does:

  rt_mutex_init_proxy_locked();
  raw_spin_unlock_irqrestore();
  rt_mutex_lock();
  rt_mutex_unlock();

vs rcu_read_unlock_special()'s RCU_READ_UNLOCK_BLOCKED branch:

  rt_mutex_unlock();


The latter looks to be unbalanced because I can't actually find a
matching lock. Also, all of that is ran with IRQs enabled. So what's the
problem?


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