[PATCH] swait: add missing barrier to swake_up

[PATCH] swait: add missing barrier to swake_up

[Nicholas Piggin]

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.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
--
I noticed this when chasing down that rcu hang bug (which
turned out to not be anything of the sort). I might be missing
something here and it's safe somehow, but if so then it should
have a comment where it diverges from normal waitqueues.

It looks like there's a few callers which are also testing
swait_active before swake_up without a barrier which look wrong,
so I must be missing something but I'm not sure what.

Thanks,
Nick
---
 kernel/sched/swait.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c
index 3d5610dcce11..9056278001d9 100644
--- a/kernel/sched/swait.c
+++ b/kernel/sched/swait.c
@@ -33,6 +33,11 @@ void swake_up(struct swait_queue_head *q)
 {
 	unsigned long flags;
 
+	/*
+	 * See waitqueue_active() comments for checking waiters outside
+	 * the lock. Same principle applies here.
+	 */
+	smp_mb();
 	if (!swait_active(q))
 		return;
 
@@ -51,6 +56,11 @@ void swake_up_all(struct swait_queue_head *q)
 	struct swait_queue *curr;
 	LIST_HEAD(tmp);
 
+	/*
+	 * See waitqueue_active() comments for checking waiters outside
+	 * the lock. Same principle applies here.
+	 */
+	smp_mb();
 	if (!swait_active(q))
 		return;
 
-- 
2.13.3

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

[Andrea Parri]

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.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> --
> I noticed this when chasing down that rcu hang bug (which
> turned out to not be anything of the sort). I might be missing
> something here and it's safe somehow, but if so then it should
> have a comment where it diverges from normal waitqueues.
> 
> It looks like there's a few callers which are also testing
> swait_active before swake_up without a barrier which look wrong,
> so I must be missing something but I'm not sure what.

Hi Nicholas. I noticed

  35a2897c2a306cca344ca5c0b43416707018f434
  ("sched/wait: Remove the lockless swait_active() check in swake_up*()")

in tip:locking/core.

  Andrea


> 
> Thanks,
> Nick
> ---
>  kernel/sched/swait.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c
> index 3d5610dcce11..9056278001d9 100644
> --- a/kernel/sched/swait.c
> +++ b/kernel/sched/swait.c
> @@ -33,6 +33,11 @@ void swake_up(struct swait_queue_head *q)
>  {
>  	unsigned long flags;
>  
> +	/*
> +	 * See waitqueue_active() comments for checking waiters outside
> +	 * the lock. Same principle applies here.
> +	 */
> +	smp_mb();
>  	if (!swait_active(q))
>  		return;
>  
> @@ -51,6 +56,11 @@ void swake_up_all(struct swait_queue_head *q)
>  	struct swait_queue *curr;
>  	LIST_HEAD(tmp);
>  
> +	/*
> +	 * See waitqueue_active() comments for checking waiters outside
> +	 * the lock. Same principle applies here.
> +	 */
> +	smp_mb();
>  	if (!swait_active(q))
>  		return;
>  
> -- 
> 2.13.3
> 

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

[Nicholas Piggin]

On Fri, 1 Sep 2017 11:23:22 +0200
Andrea Parri <parri.andrea@gmail.com> wrote:

> 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.
> > 
> > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > --
> > I noticed this when chasing down that rcu hang bug (which
> > turned out to not be anything of the sort). I might be missing
> > something here and it's safe somehow, but if so then it should
> > have a comment where it diverges from normal waitqueues.
> > 
> > It looks like there's a few callers which are also testing
> > swait_active before swake_up without a barrier which look wrong,
> > so I must be missing something but I'm not sure what.  
> 
> Hi Nicholas. I noticed
> 
>   35a2897c2a306cca344ca5c0b43416707018f434
>   ("sched/wait: Remove the lockless swait_active() check in swake_up*()")
> 
> in tip:locking/core.

Oh thanks, I missed that. Should be in 4.14/stable IMO.

As I said as well, several callers have picked up bad habits. RCU
looks okay though.

Thanks,
Nick

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

[Paul E. McKenney]

On Fri, Sep 01, 2017 at 07:55:29PM +1000, Nicholas Piggin wrote:
> On Fri, 1 Sep 2017 11:23:22 +0200
> Andrea Parri <parri.andrea@gmail.com> wrote:
> 
> > 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.
> > > 
> > > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > > --
> > > I noticed this when chasing down that rcu hang bug (which
> > > turned out to not be anything of the sort). I might be missing
> > > something here and it's safe somehow, but if so then it should
> > > have a comment where it diverges from normal waitqueues.
> > > 
> > > It looks like there's a few callers which are also testing
> > > swait_active before swake_up without a barrier which look wrong,
> > > so I must be missing something but I'm not sure what.  
> > 
> > Hi Nicholas. I noticed
> > 
> >   35a2897c2a306cca344ca5c0b43416707018f434
> >   ("sched/wait: Remove the lockless swait_active() check in swake_up*()")
> > 
> > in tip:locking/core.
> 
> Oh thanks, I missed that. Should be in 4.14/stable IMO.

This might well have been helpful to me -- I had forgotten about that
fix and am testing without it -- and suffering what look to be lost
timeouts/wakeups.  :-/

							Thanx, Paul

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@gmail.com>
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@goodmis.org>
    Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Krister Johansen <kjlx@templeofstupid.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/20170615041828.zk3a3sfyudm5p6nl@tardis
    Signed-off-by: Ingo Molnar <mingo@kernel.org>

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)) {