Re: [PATCH tip/core/rcu 3/6] rcu: Abstract dynticks extended quiescent state enter/exit operations

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
	jiangshanlai@gmail.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
	josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
	dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com,
	fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH tip/core/rcu 3/6] rcu: Abstract dynticks extended quiescent state enter/exit operations
Date: Mon, 16 Jan 2017 13:01:11 -0800	[thread overview]
Message-ID: <20170116210111.GP5238@linux.vnet.ibm.com> (raw)
In-Reply-To: <20170116164337.GB6500@twins.programming.kicks-ass.net>

On Mon, Jan 16, 2017 at 05:43:37PM +0100, Peter Zijlstra wrote:
> On Sat, Jan 14, 2017 at 12:54:42AM -0800, Paul E. McKenney wrote:
> >  /*
> > + * Record entry into an extended quiescent state.  This is only to be
> > + * called when not already in an extended quiescent state.
> > + */
> > +static void rcu_dynticks_eqs_enter(void)
> > +{
> > +	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> > +
> > +	/*
> > +	 * CPUs seeing atomic_inc() must see prior RCU read-side critical
> > +	 * sections, and we also must force ordering with the next idle
> > +	 * sojourn.
> > +	 */
> > +	smp_mb__before_atomic(); /* See above. */
> > +	atomic_inc(&rdtp->dynticks);
> > +	smp_mb__after_atomic(); /* See above. */
> > +	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > +		     atomic_read(&rdtp->dynticks) & 0x1);
> > +}
> 
> In an earlier patch you replaced things with atomic_add_return(), why
> not use atomic_inc_return() here?

The rationale is that the earlier patch uses the return value
unconditionally, while at this site, the return value would be used only
if CONFIG_RCU_EQS_DEBUG.  But mostly inertia.

							Thanx, Paul