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Date: Wed, 15 Jul 2015 11:28:01 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Daniel Wagner <daniel.wagner@bmw-carit.de>,
        Davidlohr Bueso <dave@stgolabs.net>, Ingo Molnar <mingo@redhat.com>,
        Tejun Heo <tj@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/7] rcu: Create rcu_sync infrastructure
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On Wed, Jul 15, 2015 at 08:15:11PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 15, 2015 at 11:05:46AM -0700, Paul E. McKenney wrote:
> > On Sun, Jul 12, 2015 at 01:35:48AM +0200, Oleg Nesterov wrote:
> > > It is functionally equivalent to
> > > 
> > >         struct rcu_sync_struct {
> > >                 atomic_t counter;
> > >         };
> > > 
> > >         static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss)
> > >         {
> > 
> > If you add an smp_mb() here...
> > 
> > >                 return atomic_read(&rss->counter) == 0;
> > >         }
> > > 
> > >         static inline void rcu_sync_enter(struct rcu_sync_struct *rss)
> > >         {
> > >                 atomic_inc(&rss->counter);
> > >                 synchronize_sched();
> > >         }
> > > 
> > >         static inline void rcu_sync_exit(struct rcu_sync_struct *rss)
> > >         {
> > >                 synchronize_sched();
> > 
> > You should be able to demote the above synchronize_sched() to an
> > smp_mb__before_atomic().  Even rare writes should make this tradeoff
> > worthwhile.
> 
> No, it makes the read-side primitive contain an unconditional memory
> barrier, that forgoes the entire point.
> 
> The writers are stupidly expensive already for they need global
> serialization, optimizing them in any way doesn't make sense.

That could well be the case, but it would be good to see the numbers.

							Thanx, Paul