Sparse complains that taprio_destroy() dereferences q->oper_sched and
q->admin_sched without rcu_dereference(), since they are marked as __rcu
in the taprio private structure.
1671:28: warning: incorrect type in argument 1 (different address spaces)
1671:28: expected struct callback_head *head
1671:28: got struct callback_head [noderef] __rcu *
1674:28: warning: incorrect type in argument 1 (different address spaces)
1674:28: expected struct callback_head *head
1674:28: got struct callback_head [noderef] __rcu *
To silence that build warning, do actually use rcu_dereference().
It would have been good if we had a writer-side lock to be able to use
rcu_dereference_protected(), but even the writer side taprio_change()
uses rcu_dereference() on these, only to close the critical section
immediately afterwards (and still keep using the variables).
Fixes: a3d43c0d56f1 ("taprio: Add support adding an admin schedule")
Signed-off-by: Vladimir Oltean <[email protected]>
---
net/sched/sch_taprio.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 5bffc37022e0..fbf84404408f 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -1644,6 +1644,7 @@ static void taprio_destroy(struct Qdisc *sch)
{
struct taprio_sched *q = qdisc_priv(sch);
struct net_device *dev = qdisc_dev(sch);
+ struct sched_gate_list *oper, *admin;
unsigned int i;
spin_lock(&taprio_list_lock);
@@ -1667,11 +1668,18 @@ static void taprio_destroy(struct Qdisc *sch)
netdev_reset_tc(dev);
- if (q->oper_sched)
- call_rcu(&q->oper_sched->rcu, taprio_free_sched_cb);
+ rcu_read_lock();
+
+ oper = rcu_dereference(q->oper_sched);
+ admin = rcu_dereference(q->admin_sched);
+
+ if (oper)
+ call_rcu(&oper->rcu, taprio_free_sched_cb);
- if (q->admin_sched)
- call_rcu(&q->admin_sched->rcu, taprio_free_sched_cb);
+ if (admin)
+ call_rcu(&admin->rcu, taprio_free_sched_cb);
+
+ rcu_read_unlock();
}
static int taprio_init(struct Qdisc *sch, struct nlattr *opt,
--
2.34.1
On Wed, Sep 14, 2022 at 05:34:39PM +0300, Vladimir Oltean wrote:
> - if (q->oper_sched)
> - call_rcu(&q->oper_sched->rcu, taprio_free_sched_cb);
> + rcu_read_lock();
> +
> + oper = rcu_dereference(q->oper_sched);
> + admin = rcu_dereference(q->admin_sched);
> +
> + if (oper)
> + call_rcu(&oper->rcu, taprio_free_sched_cb);
>
> - if (q->admin_sched)
> - call_rcu(&q->admin_sched->rcu, taprio_free_sched_cb);
> + if (admin)
> + call_rcu(&admin->rcu, taprio_free_sched_cb);
> +
> + rcu_read_unlock();
> }
I decided to code up this patch at the last minute, since the sparse
warning was bugging me. But after more testing (including with lockdep,
which says absolutely nothing), I notice some RCU stalls after this
patch. It looks like call_rcu() really doesn't like to be called under
rcu_read_lock(). Please discard this patch set from patchwork, I'll
resend when things will work properly.