linux-kernel - Re: [PATCH] RCU: implement rcu_read_[un]lock

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Date:	Sat, 02 Aug 2008 01:06:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Tejun Heo <tj@...nel.org>, dipankar@...ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] RCU: implement rcu_read_[un]lock_preempt()

On Fri, 2008-08-01 at 14:10 -0700, Paul E. McKenney wrote:
> On Wed, Jul 16, 2008 at 08:07:49AM +0200, Peter Zijlstra wrote:
> > On Mon, 2008-07-14 at 14:57 +0900, Tejun Heo wrote:
> > > With the introduction of preemptible RCU, RCU doesn't gurantee that
> > > its critical section runs on the CPU it started to run.  As there are
> > > cases where non-preemptible RCU critical section makes sense, create
> > > new RCU read lock variants which turns of preemption -
> > > rcu_read_[un]lock_preempt() which are identical to rcu_read_[un]lock()
> > > for classic implementation and have enclosing preempt disable/enable
> > > for preemptible RCU.
> > > 
> > > Signed-off-by: Tejun Heo <tj@...nel.org>
> > 
> > Sorry, NAK.
> > 
> > If you need preempt off you need it for other reasons than RCU, so
> > mixing it in the interface doesn't make sense to me.
> 
> What Peter said.
> 
> For example, you could simply use preempt_disable() and preempt_enable()
> to guard the read side (or wrapper these as Mathieu suggests in
> http://lkml.org/lkml/2008/7/15/605, though Mathieu has not yet
> convinced me that these wrappers are a good idea), and then use either
> call_rcu_sched() or synchronize_sched() for the update side.
> 
> To summarize:
> 
> o	Replace your rcu_read_lock_preempt() with preempt_disable().
> 
> o	Replace your rcu_read_unlock_preempt() with preempt_enable().
> 
> o	Replace your use of call_rcu() with call_rcu_sched().
> 
> o	Replace your use of synchronize_rcu() with synchronize_sched().
> 
> And then you don't need these new primitives.
> 
> However!!!  This must be done carefully, as long sections of code
> with preemption disabled are really bad for realtime response.

Right - what I said in the other mail, we should really not use the
sched-RCU variant if we can avoid it.

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