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Subject[PATCH] avoid race condition in pick_next_task_fair in kernel/sched_fair.c
From
Hi,

I walked some code in kernel/sched_fair.c in version 2.6.35-rc3, and
got the following potential failure:

static struct task_struct *pick_next_task_fair(struct rq *rq)
{
...
if (!cfs_rq->nr_running)
return NULL;

do {
se = pick_next_entity(cfs_rq);
set_next_entity(cfs_rq, se);
cfs_rq = group_cfs_rq(se);
} while (cfs_rq);
...
}

/*
* The dequeue_task method is called after nr_running is
* decreased. We remove the task from the rbtree and
* update the fair scheduling stats:
*/
static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
{
...
dequeue_entity(cfs_rq, se, flags);
...
}

static void
dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
{
...
if (se != cfs_rq->curr)
__dequeue_entity(cfs_rq, se);
account_entity_dequeue(cfs_rq, se);
update_min_vruntime(cfs_rq);
...
}

dequeue_task_fair() is used to dequeue some task, and it calls
dequeue_entity() to finish the job. While dequeue_entity() calls
__dequeue_entity() first, then calls account_entity_dequeue()
to do substraction on cfs_rq->nr_running.
Here, if one task is dequeued, then cfs_rq->nr_running will be
changed later, e.g 1 to 0.
In the pick_next_task_fair(), the if check will get passed before
nr_running is set 0 and the following while structure is executed.

do {
se = pick_next_entity(cfs_rq);
set_next_entity(cfs_rq, se);
cfs_rq = group_cfs_rq(se);
} while (cfs_rq);

We may get se set NULL here, and set_next_entity may deference
NULL pointer, which can lead to Oops.

I think some lock on the metadata can fix this issue, but we may
change plenty of code to add support for lock. I think the easist
way is just substacting nr_running before dequing tasks.

Following is my patch, please check it.


From 4fe38deac173c7777cd02096950e979749170873 Mon Sep 17 00:00:00 2001
From: Wang Sheng-Hui <crosslonelyover@gmail.com>
Date: Tue, 29 Jun 2010 14:49:05 +0800
Subject: [PATCH] avoid race condition in pick_next_task_fair in
kernel/sched_fair.c


Signed-off-by: Wang Sheng-Hui <crosslonelyover@gmail.com>
---
kernel/sched_fair.c | 4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index eed35ed..93073ff 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -823,9 +823,9 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct
sched_entity *se, int flags)

clear_buddies(cfs_rq, se);

+ account_entity_dequeue(cfs_rq, se);
if (se != cfs_rq->curr)
__dequeue_entity(cfs_rq, se);
- account_entity_dequeue(cfs_rq, se);
update_min_vruntime(cfs_rq);

/*
@@ -1059,7 +1059,7 @@ enqueue_task_fair(struct rq *rq, struct
task_struct *p, int flags)
}

/*
- * The dequeue_task method is called before nr_running is
+ * The dequeue_task method is called after nr_running is
* decreased. We remove the task from the rbtree and
* update the fair scheduling stats:
*/
--
1.6.3.3


--


Thanks and Best Regards,
shenghui


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