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

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Date:	Sat, 25 Nov 2006 00:13:00 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Paul E. McKenney" <paulmck@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

On 11/24, Alan Stern wrote:
>
> On Fri, 24 Nov 2006, Oleg Nesterov wrote:
> 
> > Ok, synchronize_xxx() passed 1 hour rcutorture test on dual P-III.
> > 
> > It behaves the same as srcu but optimized for writers. The fast path
> > for synchronize_xxx() is mutex_lock() + atomic_read() + mutex_unlock().
> > The slow path is __wait_event(), no polling. However, the reader does
> > atomic inc/dec on lock/unlock, and the counters are not per-cpu.
> > 
> > Jens, is it ok for you? Alan, Paul, what is your opinion?
> 
> Given that you aren't using per-cpu data, why not just rely on a spinlock?

I thought about this too, and we can re-use sp->wq.lock,

> Then everything will be simple and easy to verify,

xxx_read_lock() will be simpler, but not too much. synchronize_xxx() needs
some complication.

>                                                     with no need to worry 
> about atomic instructions or memory barriers.

spin_lock() + spin_unlock() doesn't imply mb(), it allows subsequent loads
to move into the the critical region.

I personally prefer this way, but may be you are right.

Oleg.

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