linux-kernel - Re: [RFC PATCH 1/1] kernel/rcu/tree.c: correct a check for grace period in progress

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Date:	Wed, 11 Jun 2014 00:23:57 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Dipankar Sarma <dipankar@...ibm.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/1] kernel/rcu/tree.c: correct a check for grace
 period in progress

Hi Paul,

On Wed, Jun 11, 2014 at 12:12 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>>       if (rnp->gpnum != rnp->completed ||
>> -         ACCESS_ONCE(rnp->gpnum) != ACCESS_ONCE(rnp->completed)) {
>> +         ACCESS_ONCE(rnp_root->gpnum) != ACCESS_ONCE(rnp_root->completed)) {
>
> At this point in the code, we are checking the current rcu_node structure,
> which might or might not be the root.  If it is not the root, we absolutely
> cannot compare against the root because we don't yet hold the root's lock.
>

I was a bit thrown by the double checking which is being done
(rnp->gpnum != rnp->complete) in that if condition. Once without
ACCESS_ONCE and one with. Is there any particular reason for this?

I now understand that we are comparing ->gpnum and ->completed of the
root node which might change from under us if we don't hold the root's
lock. I will keep looking :)

Thanks!
-- 
Pranith
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