Re: [PATCH net-next v3 7/7] mld: add mc_lock for protecting per-interface mld data

[Eric Dumazet <eric.dumazet@gmail.com>]

On 3/25/21 5:16 PM, Taehee Yoo wrote:
> The purpose of this lock is to avoid a bottleneck in the query/report
> event handler logic.
> 
> By previous patches, almost all mld data is protected by RTNL.
> So, the query and report event handler, which is data path logic
> acquires RTNL too. Therefore if a lot of query and report events
> are received, it uses RTNL for a long time.
> So it makes the control-plane bottleneck because of using RTNL.
> In order to avoid this bottleneck, mc_lock is added.
> 
> mc_lock protect only per-interface mld data and per-interface mld
> data is used in the query/report event handler logic.
> So, no longer rtnl_lock is needed in the query/report event handler logic.
> Therefore bottleneck will be disappeared by mc_lock.
> 

What testsuite have you run exactly to validate this monster patch ?

Have you used CONFIG_LOCKDEP=y / CONFIG_DEBUG_ATOMIC_SLEEP=y ?

> Suggested-by: Cong Wang <xiyou.wangcong@gmail.com>
> Signed-off-by: Taehee Yoo <ap420073@gmail.com>

[...]

>  /*
> - *	device multicast group del
> + * device multicast group del
>   */
>  int __ipv6_dev_mc_dec(struct inet6_dev *idev, const struct in6_addr *addr)
>  {
> @@ -943,8 +967,9 @@ int __ipv6_dev_mc_dec(struct inet6_dev *idev, const struct in6_addr *addr)
>  
>  	ASSERT_RTNL();
>  
> +	mutex_lock(&idev->mc_lock);
>  	for (map = &idev->mc_list;
> -	     (ma = rtnl_dereference(*map));
> +	     (ma = mc_dereference(*map, idev));
>  	     map = &ma->next) {
>  		if (ipv6_addr_equal(&ma->mca_addr, addr)) {
>  			if (--ma->mca_users == 0) {

This can be called with rcu_bh held, thus :

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:928
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 4624, name: kworker/1:2
4 locks held by kworker/1:2/4624:
 #0: ffff88802135d138 ((wq_completion)ipv6_addrconf){+.+.}-{0:0}, at: arch_atomic64_set arch/x86/include/asm/atomic64_64.h:34 [inline]
 #0: ffff88802135d138 ((wq_completion)ipv6_addrconf){+.+.}-{0:0}, at: atomic64_set include/asm-generic/atomic-instrumented.h:856 [inline]
 #0: ffff88802135d138 ((wq_completion)ipv6_addrconf){+.+.}-{0:0}, at: atomic_long_set include/asm-generic/atomic-long.h:41 [inline]
 #0: ffff88802135d138 ((wq_completion)ipv6_addrconf){+.+.}-{0:0}, at: set_work_data kernel/workqueue.c:616 [inline]
 #0: ffff88802135d138 ((wq_completion)ipv6_addrconf){+.+.}-{0:0}, at: set_work_pool_and_clear_pending kernel/workqueue.c:643 [inline]
 #0: ffff88802135d138 ((wq_completion)ipv6_addrconf){+.+.}-{0:0}, at: process_one_work+0x871/0x1600 kernel/workqueue.c:2246
 #1: ffffc90009adfda8 ((addr_chk_work).work){+.+.}-{0:0}, at: process_one_work+0x8a5/0x1600 kernel/workqueue.c:2250
 #2: ffffffff8d66d328 (rtnl_mutex){+.+.}-{3:3}, at: addrconf_verify_work+0xa/0x20 net/ipv6/addrconf.c:4572
 #3: ffffffff8bf74300 (rcu_read_lock_bh){....}-{1:2}, at: addrconf_verify_rtnl+0x2b/0x1150 net/ipv6/addrconf.c:4459
Preemption disabled at:
[<ffffffff87b39f41>] local_bh_disable include/linux/bottom_half.h:19 [inline]
[<ffffffff87b39f41>] rcu_read_lock_bh include/linux/rcupdate.h:727 [inline]
[<ffffffff87b39f41>] addrconf_verify_rtnl+0x41/0x1150 net/ipv6/addrconf.c:4461
CPU: 1 PID: 4624 Comm: kworker/1:2 Not tainted 5.12.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: ipv6_addrconf addrconf_verify_work
Call Trace:
 __dump_stack lib/dump_stack.c:79 [inline]
 dump_stack+0x141/0x1d7 lib/dump_stack.c:120
 ___might_sleep.cold+0x1f1/0x237 kernel/sched/core.c:8328
 __mutex_lock_common kernel/locking/mutex.c:928 [inline]
 __mutex_lock+0xa9/0x1120 kernel/locking/mutex.c:1096
 __ipv6_dev_mc_dec+0x5f/0x340 net/ipv6/mcast.c:970
 addrconf_leave_solict net/ipv6/addrconf.c:2182 [inline]
 addrconf_leave_solict net/ipv6/addrconf.c:2174 [inline]
 __ipv6_ifa_notify+0x5b6/0xa90 net/ipv6/addrconf.c:6077
 ipv6_ifa_notify net/ipv6/addrconf.c:6100 [inline]
 ipv6_del_addr+0x463/0xae0 net/ipv6/addrconf.c:1294
 addrconf_verify_rtnl+0xd59/0x1150 net/ipv6/addrconf.c:4488
 addrconf_verify_work+0xf/0x20 net/ipv6/addrconf.c:4573
 process_one_work+0x98d/0x1600 kernel/workqueue.c:2275
 worker_thread+0x64c/0x1120 kernel/workqueue.c:2421
 kthread+0x3b1/0x4a0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294