linux-kernel - Re: [BUG]: Crash with CONFIG_FAIR_CGROUP

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Date:	Fri, 9 Nov 2007 17:41:57 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
Cc:	sukadev@...ibm.com, balbir@...ibm.com,
	Containers <containers@...ts.osdl.org>,
	ckrm-tech@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	dhaval@...ux.vnet.ibm.com, "Ingo Molnar" <mingo@...e.hu>,
	efault@....de
Subject: Re: [BUG]: Crash with CONFIG_FAIR_CGROUP_SCHED=y

On Fri, Nov 09, 2007 at 11:59:15AM +0100, Dmitry Adamushko wrote:
> > - The second problem exposed by this test is that task_new_fair()
> >   assumes that parent and child will be part of the same group (which
> >   needn't be as this test shows). As a result, cfs_rq->curr can be NULL
> >   for the child.
> 
> Would it be better, logically-wise, to use is_same_group() instead?
> Although, we can't have 2 groups with cfs_rq->curr != NULL on the same
> CPU... so if the child belongs to another group, it's cfs_rq->curr is
> automatically NULL indeed.

Yeah ..I feel safe with an explicit !curr check, perhaps with a comment like
below added to explain when curr can be NULL?


---
 kernel/sched_fair.c |    1 +
 1 files changed, 1 insertion(+)

Index: current/kernel/sched_fair.c
===================================================================
--- current.orig/kernel/sched_fair.c
+++ current/kernel/sched_fair.c
@@ -1022,6 +1022,7 @@ static void task_new_fair(struct rq *rq,
 	update_curr(cfs_rq);
 	place_entity(cfs_rq, se, 1);
 
+	/* 'curr' will be NULL if the child belongs to a different group */
 	if (sysctl_sched_child_runs_first && this_cpu == task_cpu(p) &&
 			curr && curr->vruntime < se->vruntime) {
 		/*



-- 
Regards,
vatsa
-
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