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 <[email protected]>
--
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
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 <[email protected]>
> --
> 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
>
On Fri, 1 Sep 2017 11:23:22 +0200
Andrea Parri <[email protected]> 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 <[email protected]>
> > --
> > 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
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 <[email protected]>
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 <[email protected]>
Signed-off-by: Boqun Feng <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Krister Johansen <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Paul Gortmaker <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/20170615041828.zk3a3sfyudm5p6nl@tardis
Signed-off-by: Ingo Molnar <[email protected]>
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)) {
On Fri, Sep 01, 2017 at 07:55:29PM +1000, Nicholas Piggin wrote:
> On Fri, 1 Sep 2017 11:23:22 +0200
> Andrea Parri <[email protected]> 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 <[email protected]>
> > > --
> > > 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