Date: Fri, 6 Sep 2013 10:41:17 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, linux-kernel@vger.kernel.org,
        mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de,
        peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, darren@dvhart.com, sbw@mit.edu
Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical
 section?
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On Fri, Sep 06, 2013 at 10:21:28AM -0700, Eric Dumazet wrote:
> On Fri, 2013-09-06 at 08:18 -0700, Paul E. McKenney wrote:
> 
> > int rcu_is_cpu_idle(void)
> > {
> > 	int ret;
> > 
> > 	preempt_disable();
> > 	ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
> > 	preempt_enable();
> > 	return ret;
> > }
> 
> Paul I find this very confusing.
> 
> If caller doesn't have preemption disabled, what could be the meaning of
> this rcu_is_cpu_idle() call ?
> 
> Its result is meaningless if suddenly thread is preempted, so what is
> the point ?
> 
> Sorry if this is obvious to you.

It is a completely fair question.  In fact, this might well now be
pointing to a bug given NO_HZ_FULL.

The assumption is that if you don't have preemption disabled, you had
better be running on a CPU that RCU is paying attention to.  The rationale
involves preemptible RCU.

Suppose that you just did rcu_read_lock() on a CPU that RCU is paying
attention to.  All is well, and rcu_is_cpu_idle() will return false, as
expected.  Suppose now that it is possible to be preempted and suddenly
find yourself running on a CPU that RCU is not paying attention to.
This would have the effect of making your RCU read-side critical section
be ignored.  Therefore, it had better not be possible to be preempted
from a CPU to which RCU is paying attention to a CPU that RCU is ignoring.

So if rcu_is_cpu_idle() returns false, you had better be guaranteed
that whatever CPU you are running on (which might well be a different
one than the rcu_is_cpu_idle() was running on) is being watched by RCU.

So, Frederic, does this still work with NO_HZ_FULL?  If not, I believe
we have a bigger problem than the preempt_disable() in rcu_is_cpu_idle()!

							Thanx, Paul

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