Re: Real-Time Preemption and RCU

From: Esben Nielsen <simlo@phys.au.dk>
To: Ingo Molnar <mingo@elte.hu>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>,
	dipankar@in.ibm.com, shemminger@osdl.org, akpm@osdl.org,
	torvalds@osdl.org, rusty@au1.ibm.com, tgall@us.ibm.com,
	jim.houston@comcast.net, manfred@colorfullife.com, gh@us.ibm.com,
	linux-kernel@vger.kernel.org
Subject: Re: Real-Time Preemption and RCU
Date: Tue, 22 Mar 2005 12:39:51 +0100 (MET)	[thread overview]
Message-ID: <Pine.OSF.4.05.10503221220500.25802-100000@da410.phys.au.dk> (raw)
In-Reply-To: <20050322105640.GA28861@elte.hu>

On Tue, 22 Mar 2005, Ingo Molnar wrote:

> 
> * Esben Nielsen <simlo@phys.au.dk> wrote:
> 
> > +static inline void rcu_read_lock(void)
> > +{	
> > +	preempt_disable(); 
> > +	__get_cpu_var(rcu_data).active_readers++;
> > +	preempt_enable();
> > +}
> 
> this is buggy. Nothing guarantees that we'll do the rcu_read_unlock() on
> the same CPU, and hence ->active_readers can get out of sync.
> 

Ok, this have to be handled in the mitigration code somehow. I have already 
added an 
  current->rcu_read_depth++
so it ought to be painless. A simple solution would be not to mititagrate
threads with rcu_read_depth!=0.

> 	Ingo
> 

Esben