Re: [net-next] bpf: avoid hashtab deadlock with try_lock

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Nov 29, 2022 at 09:23:18AM -0800, 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@huawei.com> 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@gmail.com> 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

Just to be clear, I meant to refactor htab_lock_bucket() into a try
lock pattern. Also after a second thought, the below suggestion doesn't
work. I think the proper way is to make htab_lock_bucket() as a
raw_spin_trylock_irqsave().

Regards,
Boqun

> may need to introduce a virtual lockclass for ->map_locked.
> 
> Regards,
> Boqun
> 
> > Cheers,
> > Longman
> >