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>

2 files changed, 27 insertions, 27 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 5b2959b3ffc2..f88bd984df35 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 4508fe7048be..3a8fb30a91b1 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;
         }