Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

From: "Paul E. McKenney" <paulmck@us.ibm.com>
To: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Alan Stern <stern@rowland.harvard.edu>,
	Jens Axboe <jens.axboe@oracle.com>,
	Linus Torvalds <torvalds@osdl.org>,
	Thomas Gleixner <tglx@timesys.com>, Ingo Molnar <mingo@elte.hu>,
	LKML <linux-kernel@vger.kernel.org>,
	john stultz <johnstul@us.ibm.com>,
	David Miller <davem@davemloft.net>,
	Arjan van de Ven <arjan@infradead.org>,
	Andrew Morton <akpm@osdl.org>, Andi Kleen <ak@suse.de>,
	manfred@colorfullife.com
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
Date: Sun, 19 Nov 2006 21:47:26 -0800	[thread overview]
Message-ID: <20061120054725.GA4427@us.ibm.com> (raw)
In-Reply-To: <20061119222847.GA189@oleg>

On Mon, Nov 20, 2006 at 01:28:47AM +0300, Oleg Nesterov wrote:
> On 11/19, Paul E. McKenney wrote:
> >
> > On Mon, Nov 20, 2006 at 12:17:31AM +0300, Oleg Nesterov wrote:
> > >
> > > It will wait for xxx_read_unlock() on reader's side. And for this reason
> > > this idx in fact is not exactly wrong :)
> >
> > I am not seeing this.
> >
> > Let's assume sp->completed starts out zero.
> >
> > o	CPU 0 starts executing xxx_read_lock(), but is interrupted
> > 	(or whatever) just before the atomic_inc().  Upon return,
> > 	it will increment sp->ctr[0].
> 
> Right.
> 
> > o	CPU 1 executes synchronize_xxx() to completion, which it
> > 	can because CPU 0 has not yet incremented the counter.
> > 	It waited on sp->ctr[0], and incremented sp->completed to 1.
> >
> > o	CPU 0 returns from interrupt and completes xxx_read_lock(),
> > 	but has incremented sp->ctr[0].
> >
> > o	CPU 0 continues into its critical section, picking up a
> > 	pointer to an xxx-protected data structure (or, in Jens's
> > 	case starting an xxx-protected I/O).
> >
> > o	CPU 1 executes another synchronize_xxx().  This completes
> > 	immediately because it is waiting for sp->ctr[1] to go
> > 	to zero, but CPU 0 incremented sp->ctr[0].  (Right?)
> 
> Right!
> 
> > o	CPU 1 continues, either freeing a data structure while
> > 	CPU 0 is still referencing it, or, in Jens's case, completing
> > 	an I/O barrier while there is still outstanding I/O.
> >
> > Or am I missing something?
> 
> No, it is me.
> 
> Alan, Paul, thanks a lot for your patience!

No problem -- now we just need to work out if Jens's optimization is
safe, either in his situation or in general.  And I need to chase down
any remaining bugs in the patch...

							Thanx, Paul