linux-kernel - Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section?

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Date:	Mon, 9 Sep 2013 15:29:02 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, sbw@....edu
Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical
 section?

On Mon, Sep 09, 2013 at 03:14:52PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 09, 2013 at 08:55:04AM -0400, Steven Rostedt wrote:
> > On Mon, 9 Sep 2013 14:45:49 +0200
> > Frederic Weisbecker <fweisbec@...il.com> wrote:
> > 
> >  
> > > > This just proves that the caller of rcu_is_cpu_idle() must disable
> > > > preemption itself for the entire time that it needs to use the result
> > > > of rcu_is_cpu_idle().
> > > 
> > > Sorry, I don't understand your point here. What's wrong with checking the
> > > ret from another CPU?
> > 
> > Hmm, OK, this is why that code is in desperate need of a comment.
> > 
> > From reading the context a bit more, it seems that the per cpu value is
> > more a "per task" value that happens to be using per cpu variables, and
> > changes on context switches. Is that correct?
> > 
> > Anyway, it requires a comment to explain that we are not checking the
> > CPU state, but really the current task state, otherwise that 'ret'
> > value wouldn't travel with the task, but would stick with the CPU.
> 
> Egads.. and the only reason we couldn't do the immediate load is because
> of that atomic mess.
> 
> Also, if its per-task, why don't we have this in the task struct? The
> current scheme makes the context switch more expensive -- is this the
> right trade-off?

No, putting that on the task_struct won't help much in this regard I think.
Regular schedule() calls don't change that per cpu state.

Only preempt_schedule_irq() and schedule_user() are concerned with rcu eqs state
exit/restore. But still storing that on task struct won't help.

> 
> So maybe something like:
> 
> int rcu_is_cpu_idle(void)
> {
> 	/*
> 	 * Comment explaining that rcu_dynticks.dynticks really is a
> 	 * per-task something and we need preemption-safe loading.
> 	 */
> 	atomic_t dynticks = this_cpu_read(rcu_dynticks.dynticks);
> 	return !(__atomic_read(&dynticks) & 0x01);
> }
> 
> Where __atomic_read() would be like atomic_read() but without the
> volatile crap since that's entirely redundant here I think.
> 
> The this_cpu_read() should ensure we get a preemption-safe copy of the
> value.
> 
> Once that this_cpu stuff grows preemption checks we'd need something
> like __raw_this_cpu_read() or whatever the variant without preemption
> checks will be called.

Yeah I thought about using this_cpu_read() too, lets wait for the preemption
checks to get in.
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