sched/core: Fix an SMP ordering race in try_to_wake_up() vs. schedule()

Oleg noticed that its possible to falsely observe p->on_cpu == 0 such
that we'll prematurely continue with the wakeup and effectively run p on
two CPUs at the same time.

Even though the overlap is very limited; the task is in the middle of
being scheduled out; it could still result in corruption of the
scheduler data structures.

        CPU0                            CPU1

        set_current_state(...)

        <preempt_schedule>
          context_switch(X, Y)
            prepare_lock_switch(Y)
              Y->on_cpu = 1;
            finish_lock_switch(X)
              store_release(X->on_cpu, 0);

                                        try_to_wake_up(X)
                                          LOCK(p->pi_lock);

                                          t = X->on_cpu; // 0

          context_switch(Y, X)
            prepare_lock_switch(X)
              X->on_cpu = 1;
            finish_lock_switch(Y)
              store_release(Y->on_cpu, 0);
        </preempt_schedule>

        schedule();
          deactivate_task(X);
          X->on_rq = 0;

                                          if (X->on_rq) // false

                                          if (t) while (X->on_cpu)
                                            cpu_relax();

          context_switch(X, ..)
            finish_lock_switch(X)
              store_release(X->on_cpu, 0);

Avoid the load of X->on_cpu being hoisted over the X->on_rq load.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b64f163d512cbc4adb1477d7d493fd78b3571be2..7063c6a0744055c367eb82159677cfe6522b6c60 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1946,6 +1946,25 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
                goto stat;
 
 #ifdef CONFIG_SMP
+       /*
+        * Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be
+        * possible to, falsely, observe p->on_cpu == 0.
+        *
+        * One must be running (->on_cpu == 1) in order to remove oneself
+        * from the runqueue.
+        *
+        *  [S] ->on_cpu = 1;   [L] ->on_rq
+        *      UNLOCK rq->lock
+        *                      RMB
+        *      LOCK   rq->lock
+        *  [S] ->on_rq = 0;    [L] ->on_cpu
+        *
+        * Pairs with the full barrier implied in the UNLOCK+LOCK on rq->lock
+        * from the consecutive calls to schedule(); the first switching to our
+        * task, the second putting it to sleep.
+        */
+       smp_rmb();
+
        /*
         * If the owning (remote) cpu is still in the middle of schedule() with
         * this task as prev, wait until its done referencing the task.