Date: Wed, 22 Nov 2006 00:01:05 +0300
From: Oleg Nesterov <oleg@tv-sign.ru>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Alan Stern <stern@rowland.harvard.edu>,
       Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
Message-ID: <20061121210105.GA381@oleg>
References: <20061119214315.GI4427@us.ibm.com> <Pine.LNX.4.44L0.0611211244200.6140-100000@iolanthe.rowland.org> <20061121191338.GB2013@us.ibm.com>
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On 11/21, Paul E. McKenney wrote:
>
> On Tue, Nov 21, 2006 at 12:56:21PM -0500, Alan Stern wrote:
> > Here's another potential problem with the fast path approach.  It's not
> > very serious, but you might want to keep it in mind.
> > 
> > The idea is that a reader can start up on one CPU and finish on another,
> > and a writer might see the finish event but not the start event.  For
> > example:
> 
> One approach to get around this would be for the the "idx" returned from
> srcu_read_lock() to keep track of the CPU as well as the index within
> the CPU.  This would require atomic_inc()/atomic_dec() on the fast path,
> but would not add much to the overhead on x86 because the smp_mb() imposes
> an atomic operation anyway.  There would be little cache thrashing in the
> case where there is no preemption -- but if the readers almost always sleep,
> and where it is common for the srcu_read_unlock() to run on a different CPU
> than the srcu_read_lock(), then the additional cache thrashing could add
> significant overhead.

If you are going to do this, it seems better to just forget about ->per_cpu_ref,
and use only ->hardluckref[]. This also allows to avoid the polling in
synchronize_srcu().

Oleg.

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