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

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Date:	Sun, 19 Nov 2006 16:09:17 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oleg Nesterov <oleg@...sign.ru>
cc:	Jens Axboe <jens.axboe@...cle.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	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 Sun, 19 Nov 2006, Oleg Nesterov wrote:

> > What happens if synchronize_xxx manages to execute inbetween 
> > xxx_read_lock's
> > 
> >  		idx = sp->completed & 0x1;
> >  		atomic_inc(sp->ctr + idx);
> > 
> > statements?
> 
> Oops. I forgot about explicit mb() before sp->completed++ in synchronize_xxx().
> 
> So synchronize_xxx() should do
> 
> 	smp_mb();
> 	idx = sp->completed++ & 0x1;
> 
> 	for (;;) { ... }
> 
> >               You see, there's no way around using synchronize_sched().
> 
> With this change I think we are safe.
> 
> If synchronize_xxx() increments ->completed in between, the caller of
> xxx_read_lock() will see all memory ops (started before synchronize_xxx())
> completed. It is ok that synchronize_xxx() returns immediately.

Yes, the reader will see a consistent picture, but it will have 
incremented the wrong element of sp->ctr[].  What happens if another 
synchronize_xxx() occurs while the reader is still running?

Alan

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