Messages in this thread | | | Date | Wed, 11 Jun 2014 14:34:57 -0400 | From | Pranith Kumar <> | Subject | Re: [RFC PATCH 1/1] kernel/rcu/tree.c: correct a check for grace period in progress |
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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@linux.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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