linux-kernel - Re: RCU: Number of grace-periods

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Message-ID:  <loom.20090311T173526-125@post.gmane.org>
Date:	Wed, 11 Mar 2009 17:50:35 +0000 (UTC)
From:	Dmitriy V'jukov <dvyukov@...il.com>
To:	linux-kernel@...r.kernel.org
Subject:  Re: RCU: Number of grace-periods

Paul E. McKenney <paulmck <at> linux.vnet.ibm.com> writes:
> 
> Interesting thought -- but please keep in mind that acquire/release fences
> still allow subsequent stores to be reordered to precede earlier loads.
> This means that the first loads in the RCU critical section could be
> reordered to precede the final store of the rcu_read_lock() primitive.

Hmmm... Yes, I've missed this moment. I think you are right. The critical
synchronizing action of the __rcu_read_lock() is the store to the
__get_cpu_var(rcu_flipctr) (not a load!). So some code from the critical section
can hoist above the store to the __get_cpu_var(rcu_flipctr).
However the good question is how many grace-periods is required if code can
hoist above read_lock(), but can't sink below read_unlock()?
Good work for Relacy :) Nope, I've not yet tried to apply it...

--
Best regards,
Dmitriy V'jukov

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