linux-kernel - Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall

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Date:	Sun, 19 Nov 2006 13:26:19 -0800
From:	"Paul E. McKenney" <paulmck@...ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

On Sat, Nov 18, 2006 at 10:34:26PM +0300, Oleg Nesterov wrote:
> On 11/18, Paul E. McKenney wrote:
> >
> > On Sat, Nov 18, 2006 at 11:15:27AM -0500, Alan Stern wrote:
> > > > +			    smp_processor_id())->c[idx]++;
> > > > +		smp_mb();
> > > > +		preempt_enable();
> > > > +		return idx;
> > > > +	}
> > > > +	if (mutex_trylock(&sp->mutex)) {
> > > > +		preempt_enable();
> > >
> > > Move the preempt_enable() before the "if", then get rid of the
> > > preempt_enable() after the "if" block.
> >
> > No can do.  The preempt_enable() must follow the increment and
> > the memory barrier, otherwise the synchronize_sched() inside
> > synchronize_srcu() can't do its job.
> 
> Given that srcu_read_lock() does smp_mb() after ->c[idx]++, what
> is the purpose of synchronize_srcu() ? It seems to me it could be
> replaced by smp_mb().
> 
> synchronize_srcu:
> 
> 	sp->completed++;
> 
> 	mb();
> 
> 	// if the reader did any memory access _after_
> 	// srcu_read_lock()->mb() we must see the changes.
> 	while (srcu_readers_active_idx(sp, idx))
> 		sleep();
> 
> No?

I believe that this could run afoul of the example I sent out earlier
(based on Alan's example).  In my mind, the key difference between
this and Jens's suggestion is that in Jens's case, we check for -all-
the counters being zero, not just the old ones.  (But I still don't
trust Jen's optimization -- I just have not yet come up with an example
showing breakage, possibly because there isn't one, but...)

						Thanx, Paul
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