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

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Date:	Sun, 19 Nov 2006 13:07:46 -0800
From:	"Paul E. McKenney" <paulmck@...ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Linus Torvalds <torvalds@...l.org>,
	Thomas Gleixner <tglx@...esys.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>,
	David Miller <davem@...emloft.net>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@...e.de>,
	manfred@...orfullife.com
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

On Sat, Nov 18, 2006 at 09:46:24PM +0300, Oleg Nesterov wrote:
> On 11/17, Paul E. McKenney wrote:
> >
> > Oleg, any thoughts about Jens's optimization?  He would code something
> > like:
> >
> > 	if (srcu_readers_active(&my_srcu))
> > 		synchronize_srcu();
> > 	else
> > 		smp_mb();
> 
> Well, this is clearly racy, no? I am not sure, but may be we can do
> 
> 	smp_mb();
> 	if (srcu_readers_active(&my_srcu))
> 		synchronize_srcu();
> 
> in this case we also need to add 'smp_mb()' into srcu_read_lock() after
> 'atomic_inc(&sp->hardluckref)'.
> 
> > However, he is doing ordered I/O requests rather than protecting data
> > structures.
> 
> Probably this makes a difference, but I don't understand this.

OK, one hypothesis here...

	The I/Os must be somehow explicitly ordered to qualify
	for I/O-barrier separation.  If two independent processes
	issue I/Os concurrently with a third process doing an
	I/O barrier, the I/O barrier is free to separate the
	two concurrent I/Os or not, on its whim.

Jens, is the above correct?  If so, what would the two processes
need to do in order to ensure that their I/O was considered to be
ordered with respect to the I/O barrier?  Here are some possibilities:

1.	I/O barriers apply only to preceding and following I/Os from
	the process issuing the I/O barrier.

2.	As for #1 above, but restricted to task rather than process.

3.	I/O system calls that have completed are ordered by the
	barrier to precede I/O system calls that have not yet
	started, but I/O system calls still in flight could legally
	land on either side of the concurrently executing I/O
	barrier.

4.	Something else entirely?

Given some restriction like one of the above, it is entirely possible
that we don't even need the memory barrier...

						Thanx, Paul
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