Re: [PATCH] swait: add missing barrier to swake_up

[Peter Zijlstra]

On Fri, Sep 01, 2017 at 04:14:50PM +1000, Nicholas Piggin wrote:
> swake_up and swake_up_all test the swaitqueue outside the lock,
> but they are missing the barrier that would ensure visibility
> of a previous store that sets the wakeup condition with the
> load that tests the swaitqueue. This could lead to a lost wakeup
> if there is memory reordering. Fix this as prescribed by the
> waitqueue_active comments.

The below commit is in tip..

---

commit 35a2897c2a306cca344ca5c0b43416707018f434
Author: Boqun Feng <boqun.feng@xxxxxxxxx>
Date:   Thu Jun 15 12:18:28 2017 +0800

    sched/wait: Remove the lockless swait_active() check in swake_up*()
    
    Steven Rostedt reported a potential race in RCU core because of
    swake_up():
    
            CPU0                            CPU1
            ----                            ----
                                    __call_rcu_core() {
    
                                     spin_lock(rnp_root)
                                     need_wake = __rcu_start_gp() {
                                      rcu_start_gp_advanced() {
                                       gp_flags = FLAG_INIT
                                      }
                                     }
    
     rcu_gp_kthread() {
       swait_event_interruptible(wq,
            gp_flags & FLAG_INIT) {
       spin_lock(q->lock)
    
                                    *fetch wq->task_list here! *
    
       list_add(wq->task_list, q->task_list)
       spin_unlock(q->lock);
    
       *fetch old value of gp_flags here *
    
                                     spin_unlock(rnp_root)
    
                                     rcu_gp_kthread_wake() {
                                      swake_up(wq) {
                                       swait_active(wq) {
                                        list_empty(wq->task_list)
    
                                       } * return false *
    
      if (condition) * false *
        schedule();
    
    In this case, a wakeup is missed, which could cause the rcu_gp_kthread
    waits for a long time.
    
    The reason of this is that we do a lockless swait_active() check in
    swake_up(). To fix this, we can either 1) add a smp_mb() in swake_up()
    before swait_active() to provide the proper order or 2) simply remove
    the swait_active() in swake_up().
    
    The solution 2 not only fixes this problem but also keeps the swait and
    wait API as close as possible, as wake_up() doesn't provide a full
    barrier and doesn't do a lockless check of the wait queue either.
    Moreover, there are users already using swait_active() to do their quick
    checks for the wait queues, so it make less sense that swake_up() and
    swake_up_all() do this on their own.
    
    This patch then removes the lockless swait_active() check in swake_up()
    and swake_up_all().
    
    Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
    Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
    Cc: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx>
    Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
    Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
    Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
    Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
    Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
    Link: http://lkml.kernel.org/r/20170615041828.zk3a3sfyudm5p6nl@tardis
    Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>

diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c
index 3d5610dcce11..2227e183e202 100644
--- a/kernel/sched/swait.c
+++ b/kernel/sched/swait.c
@@ -33,9 +33,6 @@ void swake_up(struct swait_queue_head *q)
 {
 	unsigned long flags;
 
-	if (!swait_active(q))
-		return;
-
 	raw_spin_lock_irqsave(&q->lock, flags);
 	swake_up_locked(q);
 	raw_spin_unlock_irqrestore(&q->lock, flags);
@@ -51,9 +48,6 @@ void swake_up_all(struct swait_queue_head *q)
 	struct swait_queue *curr;
 	LIST_HEAD(tmp);
 
-	if (!swait_active(q))
-		return;
-
 	raw_spin_lock_irq(&q->lock);
 	list_splice_init(&q->task_list, &tmp);
 	while (!list_empty(&tmp)) {