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>

1 files changed, 0 insertions, 15 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 5b2959b3ffc..f88bd984df3 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;