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

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Date:	Sat, 18 Nov 2006 21:46:24 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	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 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.

Oleg.

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