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

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Message-ID: <1323167527.32012.61.camel@twins>
Date:	Tue, 06 Dec 2011 11:32:07 +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 18:05 +0800, Yong Zhang wrote:
> On Tue, Dec 06, 2011 at 10:52:32AM +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?
> 
> It could block. The issue it's legal to call rt_mutex_lock() with
> irqs disabled and we don't want might_sleep() bite us on this

Of course its legal (nobody will arrest you for it), but its also bloody
horrid.

> special case. When we are going to sleep, we re-enable irq in
> __rt_mutex_slowlock().
> 
> > 
> > > Maybe we could teach might_sleep() about this special case?
> > 
> > Sounds horrid.
> 
> Maybe, any alternative?

Maybe someone explain this mess first?
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