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 14:34:57 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	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

On 06/11/2014 02:18 PM, Paul E. McKenney wrote:
> On Tue, Jun 10, 2014 at 09:42:42PM -0700, Paul E. McKenney wrote:
>> On Wed, Jun 11, 2014 at 12:23:57AM -0400, Pranith Kumar wrote:
>>> 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 :)
>>
>> Hmmm...  Now that you mention it, that does look a bit strange.
> 
> And it turns out that you were right to begin with!  I queue your change,
> but with a full explanation in the commit log and with some additions to
> the comment.  Please see below.
> 

Awesome! A few more patches on your way :)

--
Pranith

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