Re: Bisected rcu hang (kernel/sched.c): was 2.6.33rc4 RCU hang mm spin_lock deadlock(?) after running libvirtd - reproducible.

Peter Zijlstra
2010-01-25T16:03:50Z

On Sat, 2010-01-23 at 21:49 -0500, Michael Breuer wrote:
> Libvirtd always triggers the crash; other things that fork and use mmap
> sometimes do (vsftpd, for example).

I bet its the nscgroup trainwreck, does this fix it?

---
commit fabf318e5e4bda0aca2b0d617b191884fda62703
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Thu Jan 21 21:04:57 2010 +0100

sched: Fix fork vs hotplug vs cpuset namespaces

There are a number of issues:

1) TASK_WAKING vs cgroup_clone (cpusets)

copy_process():

sched_fork()
child->state = TASK_WAKING; /* waiting for wake_up_new_task() */
if (current->nsproxy != p->nsproxy)
ns_cgroup_clone()
cgroup_clone()
mutex_lock(inode->i_mutex)
mutex_lock(cgroup_mutex)
cgroup_attach_task()
ss->can_attach()
ss->attach() [ -> cpuset_attach() ]
cpuset_attach_task()
set_cpus_allowed_ptr();
while (child->state == TASK_WAKING)
cpu_relax();
will deadlock the system.

2) cgroup_clone (cpusets) vs copy_process

So even if the above would work we still have:

copy_process():

if (current->nsproxy != p->nsproxy)
ns_cgroup_clone()
cgroup_clone()
mutex_lock(inode->i_mutex)
mutex_lock(cgroup_mutex)
cgroup_attach_task()
ss->can_attach()
ss->attach() [ -> cpuset_attach() ]
cpuset_attach_task()
set_cpus_allowed_ptr();
...

p->cpus_allowed = current->cpus_allowed

over-writing the modified cpus_allowed.

3) fork() vs hotplug

if we unplug the child's cpu after the sanity check when the child
gets attached to the task_list but before wake_up_new_task() shit
will meet with fan.

Solve all these issues by moving fork cpu selection into
wake_up_new_task().

Reported-by: Serge E. Hallyn <serue@us.ibm.com>
Tested-by: Serge E. Hallyn <serue@us.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1264106190.4283.1314.camel@laptop>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/fork.c b/kernel/fork.c
index 5b2959b..f88bd98 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1241,21 +1241,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
/* Need tasklist lock for parent etc handling! */
write_lock_irq(&tasklist_lock);

- /*
- * The task hasn't been attached yet, so its cpus_allowed mask will
- * not be changed, nor will its assigned CPU.
- *
- * The cpus_allowed mask of the parent may have changed after it was
- * copied first time - so re-copy it here, then check the child's CPU
- * to ensure it is on a valid CPU (and if not, just force it back to
- * parent's CPU). This avoids alot of nasty races.
- */
- p->cpus_allowed = current->cpus_allowed;
- p->rt.nr_cpus_allowed = current->rt.nr_cpus_allowed;
- if (unlikely(!cpu_isset(task_cpu(p), p->cpus_allowed) ||
- !cpu_online(task_cpu(p))))
- set_task_cpu(p, smp_processor_id());
-
/* CLONE_PARENT re-uses the old parent */
if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
p->real_parent = current->real_parent;
diff --git a/kernel/sched.c b/kernel/sched.c
index 4508fe7..3a8fb30 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2320,14 +2320,12 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
}

/*
- * Called from:
+ * Gets called from 3 sites (exec, fork, wakeup), since it is called without
+ * holding rq->lock we need to ensure ->cpus_allowed is stable, this is done
+ * by:
*
- * - fork, @p is stable because it isn't on the tasklist yet
- *
- * - exec, @p is unstable, retry loop
- *
- * - wake-up, we serialize ->cpus_allowed against TASK_WAKING so
- * we should be good.
+ * exec: is unstable, retry loop
+ * fork & wake-up: serialize ->cpus_allowed against TASK_WAKING
*/
static inline
int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags)
@@ -2620,9 +2618,6 @@ void sched_fork(struct task_struct *p, int clone_flags)
if (p->sched_class->task_fork)
p->sched_class->task_fork(p);

-#ifdef CONFIG_SMP
- cpu = select_task_rq(p, SD_BALANCE_FORK, 0);
-#endif
set_task_cpu(p, cpu);

#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
@@ -2652,6 +2647,21 @@ void wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
{
unsigned long flags;
struct rq *rq;
+ int cpu = get_cpu();
+
+#ifdef CONFIG_SMP
+ /*
+ * Fork balancing, do it here and not earlier because:
+ * - cpus_allowed can change in the fork path
+ * - any previously selected cpu might disappear through hotplug
+ *
+ * We still have TASK_WAKING but PF_STARTING is gone now, meaning
+ * ->cpus_allowed is stable, we have preemption disabled, meaning
+ * cpu_online_mask is stable.
+ */
+ cpu = select_task_rq(p, SD_BALANCE_FORK, 0);
+ set_task_cpu(p, cpu);
+#endif

rq = task_rq_lock(p, &flags);
BUG_ON(p->state != TASK_WAKING);
@@ -2665,6 +2675,7 @@ void wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
p->sched_class->task_woken(rq, p);
#endif
task_rq_unlock(rq, &flags);
+ put_cpu();
}

#ifdef CONFIG_PREEMPT_NOTIFIERS
@@ -7139,14 +7150,18 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
* the ->cpus_allowed mask from under waking tasks, which would be
* possible when we change rq->lock in ttwu(), so synchronize against
* TASK_WAKING to avoid that.
+ *
+ * Make an exception for freshly cloned tasks, since cpuset namespaces
+ * might move the task about, we have to validate the target in
+ * wake_up_new_task() anyway since the cpu might have gone away.
*/
again:
- while (p->state == TASK_WAKING)
+ while (p->state == TASK_WAKING && !(p->flags & PF_STARTING))
cpu_relax();

rq = task_rq_lock(p, &flags);

- if (p->state == TASK_WAKING) {
+ if (p->state == TASK_WAKING && !(p->flags & PF_STARTING)) {
task_rq_unlock(rq, &flags);
goto again;
}