linux-kernel - Re: [1/4] rcu: Detect uses of rcu read side in extended quiescent states

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Date:	Tue, 7 Jun 2011 14:58:13 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Milton Miller <miltonm@....com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [1/4] rcu: Detect uses of rcu read side in extended quiescent
 states

On Mon, Jun 06, 2011 at 09:40:05PM -0700, Paul E. McKenney wrote:> 
> The bit I am missing is how to distinguish between spinlocks (where
> sleeping is illegal) and mutexes (where sleeping is perfectly fine).
> We could teach lockdep the difference, I suppose, but it is not clear
> to me that it is worth it.

Ah, in fact it doesn't pass through any lockdep check.

It's only a function called might_sleep() that is placed in functions
that can sleep. And inside might_sleep() it checks whether it is in a preemptible
area. So it's actually locking-agnostic, it only relies on the preempt_count
and some more for the preempt rcu cases.

I think it is called CONFIG_DEBUG_SPINLOCK_SLEEP because it was first used
for spinlock debugging purposes. But then it has a broader use now: sleep
inside preemptible section, sleep inside interrupts, sleep inside rcu.

It certainly deserves a rename, like CONFIG_DEBUG_ILLEGAL_SLEEP.

> 
> In contrast, with RCU, this is straightforward -- check for rcu_sched
> and rcu_bh, but not SRCU.
> 
> 							Thanx, Paul
> 
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