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Date: Tue, 13 Jun 2023 15:02:40 +0800
From: renmingshuai <renmingshuai@...wei.com>
To: <vladbu@...dia.com>
CC: <caowangbao@...wei.com>, <davem@...emloft.net>, <edumazet@...gle.com>,
<jhs@...atatu.com>, <jiri@...nulli.us>, <kuba@...nel.org>,
<liaichun@...wei.com>, <linux-kernel@...r.kernel.org>, <liubo335@...wei.com>,
<netdev@...r.kernel.org>, <pabeni@...hat.com>, <renmingshuai@...wei.com>,
<xiyou.wangcong@...il.com>, <yanan@...wei.com>
Subject: Re: [PATCH v2] net/sched: Set the flushing flags to false to prevent an infinite loop and add one test to tdc
>On Mon 12 Jun 2023 at 22:29, renmingshuai <renmingshuai@...wei.com> wrote:
>>>On Fri 09 Jun 2023 at 17:46, renmingshuai <renmingshuai@...wei.com> wrote:
>>>> When a new chain is added by using tc, one soft lockup alarm will be
>>>> generated after delete the prio 0 filter of the chain. To reproduce
>>>> the problem, perform the following steps:
>>>> (1) tc qdisc add dev eth0 root handle 1: htb default 1
>>>> (2) tc chain add dev eth0
>>>> (3) tc filter del dev eth0 chain 0 parent 1: prio 0
>>>> (4) tc filter add dev eth0 chain 0 parent 1:
>>>>
>>>>
>>>> The refcnt of the chain added by step 2 is equal to 1. After step 3,
>>>> the flushing flag of the chain is set to true in the tcf_chain_flush()
>>>> called by tc_del_tfilter() because the prio is 0. In this case, if
>>>> we add a new filter to this chain, it will never succeed and try again
>>>> and again because the refresh flash is always true and refcnt is 1.
>>>> A soft lock alarm is generated 20 seconds later.
>>>
>>>Hi Mingshuai,
>>>
>>>Thanks for investigating and reporting this!
>>>
>>>> The stack is show as below:
>>>>
>>>> Kernel panic - not syncing: softlockup: hung tasks
>>>> CPU: 2 PID: 3321861 Comm: tc Kdump: loaded Tainted: G
>>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
>>>> Call Trace:
>>>> <IRQ>
>>>> dump_stack+0x57/0x6e
>>>> panic+0x196/0x3ec
>>>> watchdog_timer_fn.cold+0x16/0x5c
>>>> __run_hrtimer+0x5e/0x190
>>>> __hrtimer_run_queues+0x8a/0xe0
>>>> hrtimer_interrupt+0x110/0x2c0
>>>> ? irqtime_account_irq+0x49/0xf0
>>>> __sysvec_apic_timer_interrupt+0x5f/0xe0
>>>> asm_call_irq_on_stack+0x12/0x20
>>>> </IRQ>
>>>> sysvec_apic_timer_interrupt+0x72/0x80
>>>> asm_sysvec_apic_timer_interrupt+0x12/0x20
>>>> RIP: 0010:mutex_lock+0x24/0x70
>>>> RSP: 0018:ffffa836004ab9a8 EFLAGS: 00000246
>>>> RAX: 0000000000000000 RBX: ffff95bb02d76700 RCX: 0000000000000000
>>>> RDX: ffff95bb27462100 RSI: 0000000000000000 RDI: ffff95ba5b527000
>>>> RBP: ffff95ba5b527000 R08: 0000000000000001 R09: ffffa836004abbb8
>>>> R10: 000000000000000f R11: 0000000000000000 R12: 0000000000000000
>>>> R13: ffff95ba5b527000 R14: ffffa836004abbb8 R15: 0000000000000001
>>>> __tcf_chain_put+0x27/0x200
>>>> tc_new_tfilter+0x5e8/0x810
>>>> ? tc_setup_cb_add+0x210/0x210
>>>> rtnetlink_rcv_msg+0x2e3/0x380
>>>> ? rtnl_calcit.isra.0+0x120/0x120
>>>> netlink_rcv_skb+0x50/0x100
>>>> netlink_unicast+0x12d/0x1d0
>>>> netlink_sendmsg+0x286/0x490
>>>> sock_sendmsg+0x62/0x70
>>>> ____sys_sendmsg+0x24c/0x2c0
>>>> ? import_iovec+0x17/0x20
>>>> ? sendmsg_copy_msghdr+0x80/0xa0
>>>> ___sys_sendmsg+0x75/0xc0
>>>> ? do_fault_around+0x118/0x160
>>>> ? do_read_fault+0x68/0xf0
>>>> ? __handle_mm_fault+0x3f9/0x6f0
>>>> __sys_sendmsg+0x59/0xa0
>>>> do_syscall_64+0x33/0x40
>>>> entry_SYSCALL_64_after_hwframe+0x61/0xc6
>>>> RIP: 0033:0x7f96705b8247
>>>> RSP: 002b:00007ffe552e9dc8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
>>>> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f96705b8247
>>>> RDX: 0000000000000000 RSI: 00007ffe552e9e40 RDI: 0000000000000003
>>>> RBP: 0000000000000001 R08: 0000000000000001 R09: 0000558113f678b0
>>>> R10: 00007f967069ab00 R11: 0000000000000246 R12: 00000000647ea089
>>>> R13: 00007ffe552e9f30 R14: 0000000000000001 R15: 0000558113175f00
>>>>
>>>> To avoid this case, set chain->flushing to be false if the chain->refcnt
>>>> is 2 after flushing the chain when prio is 0. I also add one test to tdc.
>>>>
>>>> Fixes: 726d061286ce ("net: sched: prevent insertion of new classifiers during chain flush")
>>>> Signed-off-by: Mingshuai Ren <renmingshuai@...wei.com>
>>>> Reviewed-by: Aichun Li <Liaichun@...wei.com>
>>>> ---
>>>> V1 -> V2:
>>>> * Correct the judgment on the value chain->refcnt and add one test to tdc
>>>> ---
>>>> net/sched/cls_api.c | 7 ++++++
>>>> .../tc-testing/tc-tests/infra/filter.json | 25 +++++++++++++++++++
>>>> 2 files changed, 32 insertions(+)
>>>> create mode 100644 tools/testing/selftests/tc-testing/tc-tests/infra/filter.json
>>>>
>>>> diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
>>>> index 2621550bfddc..3ea054e03fbf 100644
>>>> --- a/net/sched/cls_api.c
>>>> +++ b/net/sched/cls_api.c
>>>> @@ -2442,6 +2442,13 @@ static int tc_del_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
>>>> tfilter_notify_chain(net, skb, block, q, parent, n,
>>>> chain, RTM_DELTFILTER, extack);
>>>> tcf_chain_flush(chain, rtnl_held);
>>>> + /* Set the flushing flags to false to prevent an infinite loop
>>>> + * when a new filter is added.
>>>> + */
>>>> + mutex_lock(&chain->filter_chain_lock);
>>>> + if (chain->refcnt == 2)
>>>> + chain->flushing = false;
>>>> + mutex_unlock(&chain->filter_chain_lock);
>>>
>>>I don't think this check is enough since there can be concurrent filter
>>>ops holding the reference to the chain. This just makes the issue harder
>>>to reproduce.
>>>
>>>I'll try to formulate a fix that takes any potential concurrent users
>>>into account and verify it with your test.
>>
>> Thanks for your reply.
>> I didn't find any concurrent scenarios that would "escape" this check.
>> That's my understanding.
>> During the flush operation, after filter_chain_lock is obtained, no new chain reference
>> could be added. After unlock, chain->flushing is set to true. The chain->refcnt and
>> chain->action_refcnt are always different. Therefore, chain->flushing will be never set to false.
>
>I agree with your analysis.
>
>> Then there seems to be no concurrency problem until flushing is set to true.
>> would you mind tell me the concurrent scenario you're talking about?
>
>What if, for example, chain->refcnt==3 after flush because there is
>another task inserting filters to the same chain concurrently? In such
>case for chains explicitly created with chains API, chain->flushing flag
>will never be reset afterwards and user will get the same lockup when
>trying to install any following filters. Also, refcnt==2 only means that
>there is no concurrent users when flushing explicitly created chains
>(because chains API holds a "passive" reference to such chains). For
>regular chains implicitly created by filters API refcnt==2 here means
>that there are active concurrent users.
Thanks, you are right.
My previous insistence that the chain->refcnt would only be increased if the filter was successfully
inserted into the chain was clearly wrong.
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