Re: [RFC][PATCH] delay rq_lock acquisition in setscheduler
From: Chris Wright (chrisw_at_osdl.org)
Date: 10/21/04
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- In reply to: Ingo Molnar: "Re: [RFC][PATCH] delay rq_lock acquisition in setscheduler"
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Date: Thu, 21 Oct 2004 10:25:28 -0700 To: Ingo Molnar <mingo@elte.hu>
* Ingo Molnar (mingo@elte.hu) wrote:
> i dont this this patch is correct, because it changes semantics by
> pushing a security-subsystem failure back into userspace. There's
> nothing wrong with two tasks trying to change a third task's policy in
> parallel - our API allows that.
Andrea and John had similar concern.
> I agree that this is a very special case of lock dependency and that the
> security subsystem should not be burdened with double-buffering messages
> just to avoid the runqueue lock on syslogd wakeup. Only this single
> scheduling-related system-call is affected by this problem.
>
> i think the right solution would be to retry the permission check if the
> policy has changed (an unlikely event). It is livelockable the same way
> seqlocks are livelockable so i'd not worry about it too much. It is also
> preemptible with PREEMPT so not a big issue. Also, the check & repeat
> code should possibly be #ifdef CONFIG_SECURITY.
I think ifdef would start to look messy in that function. This one
simply rechecks permissions if the policy has changed. Look OK?
Doing access control checks with rq_lock held can cause deadlock when
audit messages are created (via printk or audit infrastructure) which
trigger a wakeup and deadlock, as noted by both SELinux and SubDomain
folks. This patch will let the security checks happen w/out lock held,
then re-sample the p->policy in case it was raced. Originally from
John Johansen <johansen@immunix.com>, bits redone by me.
From: John Johansen <johansen@immunix.com>
Signed-off-by: Chris Wright <chrisw@osdl.org>
===== kernel/sched.c 1.367 vs edited =====
--- 1.367/kernel/sched.c 2004-10-18 22:26:52 -07:00
+++ edited/kernel/sched.c 2004-10-21 09:41:23 -07:00
@@ -3038,7 +3038,7 @@
{
struct sched_param lp;
int retval = -EINVAL;
- int oldprio;
+ int oldprio, oldpolicy = -1;
prio_array_t *array;
unsigned long flags;
runqueue_t *rq;
@@ -3060,23 +3060,17 @@
retval = -ESRCH;
if (!p)
- goto out_unlock_tasklist;
-
- /*
- * To be able to change p->policy safely, the apropriate
- * runqueue lock must be held.
- */
- rq = task_rq_lock(p, &flags);
-
+ goto out_unlock;
+recheck:
+ /* double check policy once rq lock held */
if (policy < 0)
- policy = p->policy;
+ policy = oldpolicy = p->policy;
else {
retval = -EINVAL;
if (policy != SCHED_FIFO && policy != SCHED_RR &&
policy != SCHED_NORMAL)
goto out_unlock;
}
-
/*
* Valid priorities for SCHED_FIFO and SCHED_RR are
* 1..MAX_USER_RT_PRIO-1, valid priority for SCHED_NORMAL is 0.
@@ -3098,7 +3092,17 @@
retval = security_task_setscheduler(p, policy, &lp);
if (retval)
goto out_unlock;
-
+ /*
+ * To be able to change p->policy safely, the apropriate
+ * runqueue lock must be held.
+ */
+ rq = task_rq_lock(p, &flags);
+ /* recheck policy now with rq lock held */
+ if (unlikely(oldpolicy != -1 && oldpolicy != p->policy)) {
+ policy = oldpolicy = -1;
+ task_rq_unlock(rq, &flags);
+ goto recheck;
+ }
array = p->array;
if (array)
deactivate_task(p, task_rq(p));
@@ -3118,12 +3122,9 @@
} else if (TASK_PREEMPTS_CURR(p, rq))
resched_task(rq->curr);
}
-
-out_unlock:
task_rq_unlock(rq, &flags);
-out_unlock_tasklist:
+out_unlock:
read_unlock_irq(&tasklist_lock);
-
out_nounlock:
return retval;
}
-
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- Previous message: David Lang: "Re: HARDWARE: Open-Source-Friendly Graphics Cards -- Viable?"
- In reply to: Ingo Molnar: "Re: [RFC][PATCH] delay rq_lock acquisition in setscheduler"
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
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