Sorry for the late reply, I am slowly crawling through my mbox...

On 11/30, tip-bot for Mike Galbraith wrote:
quoted text
>
> Commit-ID:  5091faa449ee0b7d73bc296a93bca9540fc51d0a
> Gitweb:     http://git.kernel.org/tip/5091faa449ee0b7d73bc296a93bca9540fc51d0a
> Author:     Mike Galbraith <efault@gmx.de>
> AuthorDate: Tue, 30 Nov 2010 14:18:03 +0100
> Committer:  Ingo Molnar <mingo@elte.hu>
> CommitDate: Tue, 30 Nov 2010 16:03:35 +0100

I assume this is the latest version. In this case I think it needs
minor fixes.

quoted text
> +#ifdef CONFIG_PROC_FS
> +
> +/* Called with siglock held. */

This is not true, and that is why we can't blindly use kref_get().

quoted text
> +int proc_sched_autogroup_set_nice(struct task_struct *p, int *nice)
> +{
> +	static unsigned long next = INITIAL_JIFFIES;
> +	struct autogroup *ag;
> +	int err;
> +
> +	if (*nice < -20 || *nice > 19)
> +		return -EINVAL;
> +
> +	err = security_task_setnice(current, *nice);
> +	if (err)
> +		return err;
> +
> +	if (*nice < 0 && !can_nice(current, *nice))
> +		return -EPERM;
> +
> +	/* this is a heavy operation taking global locks.. */
> +	if (!capable(CAP_SYS_ADMIN) && time_before(jiffies, next))
> +		return -EAGAIN;
> +
> +	next = HZ / 10 + jiffies;
> +	ag = autogroup_kref_get(p->signal->autogroup);

We can race with autogroup_move_group() and use the already freed
->autogroup. We need ->siglock or task_rq_lock() to read it.

IOW, I think we need something like the patch below, but - sorry -
if was completely untested.

And the question,

quoted text
> +	down_write(&ag->lock);
> +	err = sched_group_set_shares(ag->tg, prio_to_weight[*nice + 20]);

Do we really want this if ag == autogroup_default ? Say, autogroup_create()
fails, now the owner of this process can affect init_task_group. Or admin
can change init_task_group "by accident" (although currently this is hardly
possible, sched_autogroup_detach() has no callers). Just curious.

Oleg.

--- a/kernel/sched_autogroup.c
+++ a/kernel/sched_autogroup.c
@@ -41,6 +41,19 @@ static inline struct autogroup *autogrou
 	return ag;
 }
 
+static struct autogroup *task_get_autogroup(struct task_struct *p)
+{
+	struct autogroup *ag = NULL;
+	unsigned long flags;
+
+	if (lock_task_sighand(p, &flags)) {
+		ag = autogroup_kref_get(p->signal->autogroup);
+		unlock_task_sighand(p, &flags);
+	}
+
+	return ag;
+}
+
 static inline struct autogroup *autogroup_create(void)
 {
 	struct autogroup *ag = kzalloc(sizeof(*ag), GFP_KERNEL);
@@ -149,11 +162,7 @@ EXPORT_SYMBOL(sched_autogroup_detach);
 
 void sched_autogroup_fork(struct signal_struct *sig)
 {
-	struct task_struct *p = current;
-
-	spin_lock_irq(&p->sighand->siglock);
-	sig->autogroup = autogroup_kref_get(p->signal->autogroup);
-	spin_unlock_irq(&p->sighand->siglock);
+	sig->autogroup = task_get_autogroup(current);
 }
 
 void sched_autogroup_exit(struct signal_struct *sig)
@@ -172,7 +181,6 @@ __setup("noautogroup", setup_autogroup);
 
 #ifdef CONFIG_PROC_FS
 
-/* Called with siglock held. */
 int proc_sched_autogroup_set_nice(struct task_struct *p, int *nice)
 {
 	static unsigned long next = INITIAL_JIFFIES;
@@ -193,9 +201,11 @@ int proc_sched_autogroup_set_nice(struct
 	if (!capable(CAP_SYS_ADMIN) && time_before(jiffies, next))
 		return -EAGAIN;
 
-	next = HZ / 10 + jiffies;
-	ag = autogroup_kref_get(p->signal->autogroup);
+	ag = task_get_autogroup(p);
+	if (!ag)
+		return -ESRCH;
 
+	next = HZ / 10 + jiffies;
 	down_write(&ag->lock);
 	err = sched_group_set_shares(ag->tg, prio_to_weight[*nice + 20]);
 	if (!err)
@@ -209,7 +219,10 @@ int proc_sched_autogroup_set_nice(struct
 
 void proc_sched_autogroup_show_task(struct task_struct *p, struct seq_file *m)
 {
-	struct autogroup *ag = autogroup_kref_get(p->signal->autogroup);
+	struct autogroup *ag = task_get_autogroup(p);
+
+	if (!ag)
+		return;
 
 	down_read(&ag->lock);
 	seq_printf(m, "/autogroup-%ld nice %d\n", ag->id, ag->nice);

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