Re: [net-next] bpf: avoid hashtab deadlock with try_lock
From: Hou Tao
Date: Tue Nov 29 2022 - 20:37:18 EST
Hi,
On 11/30/2022 1:23 AM, Boqun Feng wrote:
> On Tue, Nov 29, 2022 at 11:06:51AM -0500, Waiman Long wrote:
>> On 11/29/22 07:45, Hou Tao wrote:
>>> Hi,
>>>
>>> On 11/29/2022 2:06 PM, Tonghao Zhang wrote:
>>>> On Tue, Nov 29, 2022 at 12:32 PM Hou Tao <houtao1@xxxxxxxxxx> wrote:
>>>>> Hi,
>>>>>
>>>>> On 11/29/2022 5:55 AM, Hao Luo wrote:
>>>>>> On Sun, Nov 27, 2022 at 7:15 PM Tonghao Zhang <xiangxia.m.yue@xxxxxxxxx> wrote:
>>>>>> Hi Tonghao,
>>>>>>
>>>>>> With a quick look at the htab_lock_bucket() and your problem
>>>>>> statement, I agree with Hou Tao that using hash &
>>>>>> min(HASHTAB_MAP_LOCK_MASK, n_bucket - 1) to index in map_locked seems
>>>>>> to fix the potential deadlock. Can you actually send your changes as
>>>>>> v2 so we can take a look and better help you? Also, can you explain
>>>>>> your solution in your commit message? Right now, your commit message
>>>>>> has only a problem statement and is not very clear. Please include
>>>>>> more details on what you do to fix the issue.
>>>>>>
>>>>>> Hao
>>>>> It would be better if the test case below can be rewritten as a bpf selftests.
>>>>> Please see comments below on how to improve it and reproduce the deadlock.
>>>>>>> Hi
>>>>>>> only a warning from lockdep.
>>>>> Thanks for your details instruction. I can reproduce the warning by using your
>>>>> setup. I am not a lockdep expert, it seems that fixing such warning needs to set
>>>>> different lockdep class to the different bucket. Because we use map_locked to
>>>>> protect the acquisition of bucket lock, so I think we can define lock_class_key
>>>>> array in bpf_htab (e.g., lockdep_key[HASHTAB_MAP_LOCK_COUNT]) and initialize the
>>>>> bucket lock accordingly.
>>> The proposed lockdep solution doesn't work. Still got lockdep warning after
>>> that, so cc +locking expert +lkml.org for lockdep help.
>>>
>>> Hi lockdep experts,
>>>
>>> We are trying to fix the following lockdep warning from bpf subsystem:
>>>
>>> [ 36.092222] ================================
>>> [ 36.092230] WARNING: inconsistent lock state
>>> [ 36.092234] 6.1.0-rc5+ #81 Tainted: G E
>>> [ 36.092236] --------------------------------
>>> [ 36.092237] inconsistent {INITIAL USE} -> {IN-NMI} usage.
>>> [ 36.092238] perf/1515 [HC1[1]:SC0[0]:HE0:SE1] takes:
>>> [ 36.092242] ffff888341acd1a0 (&htab->lockdep_key){....}-{2:2}, at:
>>> htab_lock_bucket+0x4d/0x58
>>> [ 36.092253] {INITIAL USE} state was registered at:
>>> [ 36.092255] mark_usage+0x1d/0x11d
>>> [ 36.092262] __lock_acquire+0x3c9/0x6ed
>>> [ 36.092266] lock_acquire+0x23d/0x29a
>>> [ 36.092270] _raw_spin_lock_irqsave+0x43/0x7f
>>> [ 36.092274] htab_lock_bucket+0x4d/0x58
>>> [ 36.092276] htab_map_delete_elem+0x82/0xfb
>>> [ 36.092278] map_delete_elem+0x156/0x1ac
>>> [ 36.092282] __sys_bpf+0x138/0xb71
>>> [ 36.092285] __do_sys_bpf+0xd/0x15
>>> [ 36.092288] do_syscall_64+0x6d/0x84
>>> [ 36.092291] entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>> [ 36.092295] irq event stamp: 120346
>>> [ 36.092296] hardirqs last enabled at (120345): [<ffffffff8180b97f>]
>>> _raw_spin_unlock_irq+0x24/0x39
>>> [ 36.092299] hardirqs last disabled at (120346): [<ffffffff81169e85>]
>>> generic_exec_single+0x40/0xb9
>>> [ 36.092303] softirqs last enabled at (120268): [<ffffffff81c00347>]
>>> __do_softirq+0x347/0x387
>>> [ 36.092307] softirqs last disabled at (120133): [<ffffffff810ba4f0>]
>>> __irq_exit_rcu+0x67/0xc6
>>> [ 36.092311]
>>> [ 36.092311] other info that might help us debug this:
>>> [ 36.092312] Possible unsafe locking scenario:
>>> [ 36.092312]
>>> [ 36.092313] CPU0
>>> [ 36.092313] ----
>>> [ 36.092314] lock(&htab->lockdep_key);
>>> [ 36.092315] <Interrupt>
>>> [ 36.092316] lock(&htab->lockdep_key);
>>> [ 36.092318]
>>> [ 36.092318] *** DEADLOCK ***
>>> [ 36.092318]
>>> [ 36.092318] 3 locks held by perf/1515:
>>> [ 36.092320] #0: ffff8881b9805cc0 (&cpuctx_mutex){+.+.}-{4:4}, at:
>>> perf_event_ctx_lock_nested+0x8e/0xba
>>> [ 36.092327] #1: ffff8881075ecc20 (&event->child_mutex){+.+.}-{4:4}, at:
>>> perf_event_for_each_child+0x35/0x76
>>> [ 36.092332] #2: ffff8881b9805c20 (&cpuctx_lock){-.-.}-{2:2}, at:
>>> perf_ctx_lock+0x12/0x27
>>> [ 36.092339]
>>> [ 36.092339] stack backtrace:
>>> [ 36.092341] CPU: 0 PID: 1515 Comm: perf Tainted: G E
>>> 6.1.0-rc5+ #81
>>> [ 36.092344] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>>> rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
>>> [ 36.092349] Call Trace:
>>> [ 36.092351] <NMI>
>>> [ 36.092354] dump_stack_lvl+0x57/0x81
>>> [ 36.092359] lock_acquire+0x1f4/0x29a
>>> [ 36.092363] ? handle_pmi_common+0x13f/0x1f0
>>> [ 36.092366] ? htab_lock_bucket+0x4d/0x58
>>> [ 36.092371] _raw_spin_lock_irqsave+0x43/0x7f
>>> [ 36.092374] ? htab_lock_bucket+0x4d/0x58
>>> [ 36.092377] htab_lock_bucket+0x4d/0x58
>>> [ 36.092379] htab_map_update_elem+0x11e/0x220
>>> [ 36.092386] bpf_prog_f3a535ca81a8128a_bpf_prog2+0x3e/0x42
>>> [ 36.092392] trace_call_bpf+0x177/0x215
>>> [ 36.092398] perf_trace_run_bpf_submit+0x52/0xaa
>>> [ 36.092403] ? x86_pmu_stop+0x97/0x97
>>> [ 36.092407] perf_trace_nmi_handler+0xb7/0xe0
>>> [ 36.092415] nmi_handle+0x116/0x254
>>> [ 36.092418] ? x86_pmu_stop+0x97/0x97
>>> [ 36.092423] default_do_nmi+0x3d/0xf6
>>> [ 36.092428] exc_nmi+0xa1/0x109
>>> [ 36.092432] end_repeat_nmi+0x16/0x67
>>> [ 36.092436] RIP: 0010:wrmsrl+0xd/0x1b
>> So the lock is really taken in a NMI context. In general, we advise again
>> using lock in a NMI context unless it is a lock that is used only in that
>> context. Otherwise, deadlock is certainly a possibility as there is no way
>> to mask off again NMI.
>>
> I think here they use a percpu counter as an "outer lock" to make the
> accesses to the real lock exclusive:
>
> preempt_disable();
> a = __this_cpu_inc(->map_locked);
> if (a != 1) {
> __this_cpu_dec(->map_locked);
> preempt_enable();
> return -EBUSY;
> }
> preempt_enable();
> return -EBUSY;
>
> raw_spin_lock_irqsave(->raw_lock);
>
> and lockdep is not aware that ->map_locked acts as a lock.
>
> However, I feel this may be just a reinvented try_lock pattern, Hou Tao,
> could you see if this can be refactored with a try_lock? Otherwise, you
> may need to introduce a virtual lockclass for ->map_locked.
As said by Hao Luo, the problem of using trylock in nmi context is that it can
not distinguish between dead-lock and lock with high-contention. And map_locked
is still needed even trylock is used in NMI because htab_map_update_elem() may
be reentered in a normal context through attaching a bpf program to one function
called after taken the lock. So introducing a virtual lockclass for ->map_locked
is a better idea.
Thanks,
Tao
> Regards,
> Boqun
>
>> Cheers,
>> Longman
>>
> .