[patch net-next] sched: sch_api: add missing rcu read lock to silence the warning

[Jiri Pirko]

From: Jiri Pirko <j...@mellanox.com>

In case the qdisc_match_from_root function() is called from non-rcu path
with rtnl mutex held, a suspiciout rcu usage warning appears:

[  241.504354] =============================
[  241.504358] WARNING: suspicious RCU usage
[  241.504366] 5.8.0-rc4-custom-01521-g72a7c7d549c3 #32 Not tainted
[  241.504370] -----------------------------
[  241.504378] net/sched/sch_api.c:270 RCU-list traversed in non-reader 
section!!
[  241.504382]
               other info that might help us debug this:
[  241.504388]
               rcu_scheduler_active = 2, debug_locks = 1
[  241.504394] 1 lock held by tc/1391:
[  241.504398]  #0: ffffffff85a27850 (rtnl_mutex){+.+.}-{3:3}, at: 
rtnetlink_rcv_msg+0x49a/0xbd0
[  241.504431]
               stack backtrace:
[  241.504440] CPU: 0 PID: 1391 Comm: tc Not tainted 
5.8.0-rc4-custom-01521-g72a7c7d549c3 #32
[  241.504446] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
1.13.0-2.fc32 04/01/2014
[  241.504453] Call Trace:
[  241.504465]  dump_stack+0x100/0x184
[  241.504482]  lockdep_rcu_suspicious+0x153/0x15d
[  241.504499]  qdisc_match_from_root+0x293/0x350

Fix this by taking the rcu_lock for qdisc_hash iteration.

Reported-by: Ido Schimmel <ido...@mellanox.com>
Signed-off-by: Jiri Pirko <j...@mellanox.com>
---
 net/sched/sch_api.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index 11ebba60da3b..c7cfd8dc6a77 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -267,10 +267,12 @@ static struct Qdisc *qdisc_match_from_root(struct Qdisc 
*root, u32 handle)
            root->handle == handle)
                return root;
 
+       rcu_read_lock();
        hash_for_each_possible_rcu(qdisc_dev(root)->qdisc_hash, q, hash, 
handle) {
                if (q->handle == handle)
                        return q;
        }
+       rcu_read_unlock();
        return NULL;
 }
 
-- 
2.21.3