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 <redacted>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>

[...]

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