net: sched: sch_htb: don't call qdisc_put() while holding tree lock

Recent changes that removed rtnl dependency from rules update path of tc
also made tcf_block_put() function sleeping. This function is called from
ops->destroy() of several Qdisc implementations, which in turn is called by
qdisc_put(). Some Qdiscs call qdisc_put() while holding sch tree spinlock,
which results sleeping-while-atomic BUG.

Steps to reproduce for htb:

tc qdisc add dev ens1f0 root handle 1: htb default 12
tc class add dev ens1f0 parent 1: classid 1:1 htb rate 100kbps ceil 100kbps
tc qdisc add dev ens1f0 parent 1:1 handle 40: sfq perturb 10
tc class add dev ens1f0 parent 1:1 classid 1:2 htb rate 100kbps ceil 100kbps

Resulting dmesg:

[ 4791.148551] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:909
[ 4791.151354] in_atomic(): 1, irqs_disabled(): 0, pid: 27273, name: tc
[ 4791.152805] INFO: lockdep is turned off.
[ 4791.153605] CPU: 19 PID: 27273 Comm: tc Tainted: G W 5.3.0-rc8+ #721
[ 4791.154336] Hardware name: Supermicro SYS-2028TP-DECR/X10DRT-P, BIOS 2.0b 03/30/2017
[ 4791.155075] Call Trace:
[ 4791.155803] dump_stack+0x85/0xc0
[ 4791.156529] ___might_sleep.cold+0xac/0xbc
[ 4791.157251] __mutex_lock+0x5b/0x960
[ 4791.157966] ? console_unlock+0x363/0x5d0
[ 4791.158676] ? tcf_chain0_head_change_cb_del.isra.0+0x1b/0xf0
[ 4791.159395] ? tcf_chain0_head_change_cb_del.isra.0+0x1b/0xf0
[ 4791.160103] tcf_chain0_head_change_cb_del.isra.0+0x1b/0xf0
[ 4791.160815] tcf_block_put_ext.part.0+0x21/0x50
[ 4791.161530] tcf_block_put+0x50/0x70
[ 4791.162233] sfq_destroy+0x15/0x50 [sch_sfq]
[ 4791.162936] qdisc_destroy+0x5f/0x160
[ 4791.163642] htb_change_class.cold+0x5df/0x69d [sch_htb]
[ 4791.164505] tc_ctl_tclass+0x19d/0x480
[ 4791.165360] rtnetlink_rcv_msg+0x170/0x4b0
[ 4791.166191] ? netlink_deliver_tap+0x95/0x400
[ 4791.166907] ? rtnl_dellink+0x2d0/0x2d0
[ 4791.167625] netlink_rcv_skb+0x49/0x110
[ 4791.168345] netlink_unicast+0x171/0x200
[ 4791.169058] netlink_sendmsg+0x224/0x3f0
[ 4791.169771] sock_sendmsg+0x5e/0x60
[ 4791.170475] ___sys_sendmsg+0x2ae/0x330
[ 4791.171183] ? ___sys_recvmsg+0x159/0x1f0
[ 4791.171894] ? do_wp_page+0x9c/0x790
[ 4791.172595] ? __handle_mm_fault+0xcd3/0x19e0
[ 4791.173309] __sys_sendmsg+0x59/0xa0
[ 4791.174024] do_syscall_64+0x5c/0xb0
[ 4791.174725] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 4791.175435] RIP: 0033:0x7f0aa41497b8
[ 4791.176129] Code: 89 02 48 c7 c0 ff ff ff ff eb bb 0f 1f 80 00 00 00 00 f3 0f 1e fa 48 8d 05 65 8f 0c 00 8b 00 85 c0 75 17 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 58 c3 0f 1f 80 00 00 00 00 48 83 ec 28 89 5
4
[ 4791.177532] RSP: 002b:00007fff4e37d588 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
[ 4791.178243] RAX: ffffffffffffffda RBX: 000000005d8132f7 RCX: 00007f0aa41497b8
[ 4791.178947] RDX: 0000000000000000 RSI: 00007fff4e37d5f0 RDI: 0000000000000003
[ 4791.179662] RBP: 0000000000000000 R08: 0000000000000001 R09: 00000000020149a0
[ 4791.180382] R10: 0000000000404eda R11: 0000000000000246 R12: 0000000000000001
[ 4791.181100] R13: 000000000047f640 R14: 0000000000000000 R15: 0000000000000000

In htb_change_class() function save parent->leaf.q to local temporary
variable and put reference to it after sch tree lock is released in order
not to call potentially sleeping cls API in atomic section. This is safe to
do because Qdisc has already been reset by qdisc_purge_queue() inside sch
tree lock critical section.

Fixes: c266f64dbfa2 ("net: sched: protect block state with mutex")
Signed-off-by: Vlad Buslov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sched/sch_htb.c

···
 	struct htb_class *cl = (struct htb_class *)*arg, *parent;
 	struct nlattr *opt = tca[TCA_OPTIONS];
 	struct nlattr *tb[TCA_HTB_MAX + 1];
+	struct Qdisc *parent_qdisc = NULL;
 	struct tc_htb_opt *hopt;
 	u64 rate64, ceil64;
 	int warn = 0;
···
 		if (parent && !parent->level) {
 			/* turn parent into inner node */
 			qdisc_purge_queue(parent->leaf.q);
-			qdisc_put(parent->leaf.q);
+			parent_qdisc = parent->leaf.q;
 			if (parent->prio_activity)
 				htb_deactivate(q, parent);
 
···
 	cl->cbuffer = PSCHED_TICKS2NS(hopt->cbuffer);
 
 	sch_tree_unlock(sch);
+	qdisc_put(parent_qdisc);
 
 	if (warn)
 		pr_warn("HTB: quantum of class %X is %s. Consider r2q change.\n",