On Thu, 2010-01-21 at 18:35 +0100, Peter Zijlstra wrote:
> On Thu, 2010-01-21 at 11:13 -0600, Serge E. Hallyn wrote:
> > The culprit is e2912009fb7b715728311b0d8fe327a1432b3f79
> > sched: Ensure set_task_cpu() is never called on blocked tasks
> >
> > If you mount both the ns and cpuset cgroups with this patch applied,
> > then doing clone with CLONE_NEWPID, CLONE_NEWNET, etc, you get the
> > hang. The hang is actually hard enough that alt-sysrq isn't helpful :)
> > Still trying to figure out what is going on - Peter, any ideas offhand?
> Hmm, I have an idea.. does it really need the ns cgroup stuff?
It appears so, what is happening is that:
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();
which will pretty much mess up your system. However sysrq not working
shouldn't be amongst that and s390 working is also unexplained.
While that spin on TASK_WAKING is new, the behaviour was dubious because
when we stick the new child into the tasklist we copy the
parent->cpus_allowed back into the child:
/* 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());
which mostly stems from 2005.
Now, I'm not quite sure how to fix this to be honest.. calling
->can_attach and ->attach on half finished tasks seems somewhat ill
defined.
So while pondering this I think I might have found another bug which
needs to get closed, hotplug vs fork might end up removing the target
cpu between this second ->cpus_allowed copy in copy_process() and
wake_up_new_task(), so we need to move the fork migration step anyway.
So I think the below ought to cure at least the immediate problem,
leaving the question if cgroups really want to call ->attach that early.
Utterly untested.... never even seen a compiler but here goes:
---
Subject: sched: Fix fork vs hotplug vs cpuset namespaces
From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Date: Thu Jan 21 21:04:57 CET 2010
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@xxxxxxxxxx>
Almost-Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
kernel/fork.c | 15 ---------------
kernel/sched.c | 39 +++++++++++++++++++++++++++------------
2 files changed, 27 insertions(+), 27 deletions(-)
Index: linux-2.6/kernel/fork.c
===================================================================
--- linux-2.6.orig/kernel/fork.c
+++ linux-2.6/kernel/fork.c
@@ -1241,21 +1241,6 @@ static struct task_struct *copy_process(
/* 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;
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -2300,14 +2300,12 @@ static int select_fallback_rq(int cpu, s
}
/*
- * 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)
@@ -2600,9 +2598,6 @@ void sched_fork(struct task_struct *p, i
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)
@@ -2632,6 +2627,21 @@ void wake_up_new_task(struct task_struct
{
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);
@@ -2645,6 +2655,7 @@ void wake_up_new_task(struct task_struct
p->sched_class->task_woken(rq, p);
#endif
task_rq_unlock(rq, &flags);
+ put_cpu();
}
#ifdef CONFIG_PREEMPT_NOTIFIERS
@@ -5316,14 +5327,18 @@ int set_cpus_allowed_ptr(struct task_str
* 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;
}
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