sched: Ensure set_task_cpu() is never called on blocked tasks

In order to clean up the set_task_cpu() rq dependencies we need
to ensure it is never called on blocked tasks because such usage
does not pair with consistent rq->lock usage.

This puts the migration burden on ttwu().

Furthermore we need to close a race against changing
->cpus_allowed, since select_task_rq() runs with only preemption
disabled.

For sched_fork() this is safe because the child isn't in the
tasklist yet, for wakeup we fix this by synchronizing
set_cpus_allowed_ptr() against TASK_WAKING, which leaves
sched_exec to be a problem

This also closes a hole in (6ad4c1888 sched: Fix balance vs
hotplug race) where ->select_task_rq() doesn't validate the
result against the sched_domain/root_domain.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <20091216170517.807938893@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 66 insertions, 19 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 1672823aabfe..33d7965f63f0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2018,22 +2018,15 @@ static inline void check_class_changed(struct rq *rq, struct task_struct *p,
  */
 void kthread_bind(struct task_struct *p, unsigned int cpu)
 {
-        struct rq *rq = cpu_rq(cpu);
-        unsigned long flags;
-
         /* Must have done schedule() in kthread() before we set_task_cpu */
         if (!wait_task_inactive(p, TASK_UNINTERRUPTIBLE)) {
                 WARN_ON(1);
                 return;
         }
 
-        raw_spin_lock_irqsave(&rq->lock, flags);
-        update_rq_clock(rq);
-        set_task_cpu(p, cpu);
         p->cpus_allowed = cpumask_of_cpu(cpu);
         p->rt.nr_cpus_allowed = 1;
         p->flags |= PF_THREAD_BOUND;
-        raw_spin_unlock_irqrestore(&rq->lock, flags);
 }
 EXPORT_SYMBOL(kthread_bind);
 
@@ -2074,6 +2067,14 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
         struct cfs_rq *old_cfsrq = task_cfs_rq(p),
                       *new_cfsrq = cpu_cfs_rq(old_cfsrq, new_cpu);
 
+#ifdef CONFIG_SCHED_DEBUG
+        /*
+         * We should never call set_task_cpu() on a blocked task,
+         * ttwu() will sort out the placement.
+         */
+        WARN_ON(p->state != TASK_RUNNING && p->state != TASK_WAKING);
+#endif
+
         trace_sched_migrate_task(p, new_cpu);
 
         if (old_cpu != new_cpu) {
@@ -2107,13 +2108,10 @@ migrate_task(struct task_struct *p, int dest_cpu, struct migration_req *req)
 
         /*
          * If the task is not on a runqueue (and not running), then
-         * it is sufficient to simply update the task's cpu field.
+         * the next wake-up will properly place the task.
          */
-        if (!p->se.on_rq && !task_running(rq, p)) {
-                update_rq_clock(rq);
-                set_task_cpu(p, dest_cpu);
+        if (!p->se.on_rq && !task_running(rq, p))
                 return 0;
-        }
 
         init_completion(&req->done);
         req->task = p;
@@ -2319,10 +2317,42 @@ void task_oncpu_function_call(struct task_struct *p,
 }
 
 #ifdef CONFIG_SMP
+/*
+ * Called from:
+ *
+ *  - fork, @p is stable because it isn't on the tasklist yet
+ *
+ *  - exec, @p is unstable XXX
+ *
+ *  - wake-up, we serialize ->cpus_allowed against TASK_WAKING so
+ *             we should be good.
+ */
 static inline
 int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags)
 {
-        return p->sched_class->select_task_rq(p, sd_flags, wake_flags);
+        int cpu = p->sched_class->select_task_rq(p, sd_flags, wake_flags);
+
+        /*
+         * In order not to call set_task_cpu() on a blocking task we need
+         * to rely on ttwu() to place the task on a valid ->cpus_allowed
+         * cpu.
+         *
+         * Since this is common to all placement strategies, this lives here.
+         *
+         * [ this allows ->select_task() to simply return task_cpu(p) and
+         *   not worry about this generic constraint ]
+         */
+        if (unlikely(!cpumask_test_cpu(cpu, &p->cpus_allowed) ||
+                     !cpu_active(cpu))) {
+
+                cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
+                /*
+                 * XXX: race against hot-plug modifying cpu_active_mask
+                 */
+                BUG_ON(cpu >= nr_cpu_ids);
+        }
+
+        return cpu;
 }
 #endif
 
@@ -7098,7 +7128,23 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
         struct rq *rq;
         int ret = 0;
 
+        /*
+         * Since we rely on wake-ups to migrate sleeping tasks, don't change
+         * 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.
+         */
+again:
+        while (p->state == TASK_WAKING)
+                cpu_relax();
+
         rq = task_rq_lock(p, &flags);
+
+        if (p->state == TASK_WAKING) {
+                task_rq_unlock(rq, &flags);
+                goto again;
+        }
+
         if (!cpumask_intersects(new_mask, cpu_active_mask)) {
                 ret = -EINVAL;
                 goto out;
@@ -7154,7 +7200,7 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);
 static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
 {
         struct rq *rq_dest, *rq_src;
-        int ret = 0, on_rq;
+        int ret = 0;
 
         if (unlikely(!cpu_active(dest_cpu)))
                 return ret;
@@ -7170,12 +7216,13 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
         if (!cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
                 goto fail;
 
-        on_rq = p->se.on_rq;
-        if (on_rq)
+        /*
+         * If we're not on a rq, the next wake-up will ensure we're
+         * placed properly.
+         */
+        if (p->se.on_rq) {
                 deactivate_task(rq_src, p, 0);
-
-        set_task_cpu(p, dest_cpu);
-        if (on_rq) {
+                set_task_cpu(p, dest_cpu);
                 activate_task(rq_dest, p, 0);
                 check_preempt_curr(rq_dest, p, 0);
         }