Re: [PATCH] kfence: Avoid stalling work queue task without allocations

Boqun Feng
2020-11-24T03:01:11Z

Hi Steven,

On Mon, Nov 23, 2020 at 01:42:27PM -0500, Steven Rostedt wrote:
> On Mon, 23 Nov 2020 11:28:12 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > I noticed:
> >
> >
> > [ 237.650900] enabling event benchmark_event
> >
> > In both traces. Could you disable CONFIG_TRACEPOINT_BENCHMARK and see if
> > the issue goes away. That event kicks off a thread that spins in a tight
> > loop for some time and could possibly cause some issues.
> >
> > It still shouldn't break things, we can narrow it down if it is the culprit.
>
> [ Added Thomas ]
>
> And that's just one issue. I don't think that has anything to do with the
> other one:
>
> [ 1614.162007] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> [ 1614.168625] (detected by 0, t=3752 jiffies, g=3529, q=1)
> [ 1614.170825] rcu: All QSes seen, last rcu_preempt kthread activity 242 (4295293115-4295292873), jiffies_till_next_fqs=1, root ->qsmask 0x0
> [ 1614.194272]
> [ 1614.196673] ================================
> [ 1614.199738] WARNING: inconsistent lock state
> [ 1614.203056] 5.10.0-rc4-next-20201119-00004-g77838ee21ff6-dirty #21 Not tainted
> [ 1614.207012] --------------------------------
> [ 1614.210125] inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage.
> [ 1614.213832] swapper/0/1 [HC0[0]:SC0[0]:HE0:SE1] takes:
> [ 1614.217288] ffffd942547f47d8 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x7c0/0x17a0
> [ 1614.225496] {IN-HARDIRQ-W} state was registered at:
> [ 1614.229031] __lock_acquire+0xae8/0x1ac8
> [ 1614.232203] lock_acquire+0x268/0x508
> [ 1614.235254] _raw_spin_lock_irqsave+0x78/0x14c
> [ 1614.238547] rcu_sched_clock_irq+0x7c0/0x17a0
> [ 1614.241757] update_process_times+0x6c/0xb8
> [ 1614.244950] tick_sched_handle.isra.0+0x58/0x88
> [ 1614.248225] tick_sched_timer+0x68/0xe0
> [ 1614.251304] __hrtimer_run_queues+0x288/0x730
> [ 1614.254516] hrtimer_interrupt+0x114/0x288
> [ 1614.257650] arch_timer_handler_virt+0x50/0x70
> [ 1614.260922] handle_percpu_devid_irq+0x104/0x4c0
> [ 1614.264236] generic_handle_irq+0x54/0x78
> [ 1614.267385] __handle_domain_irq+0xac/0x130
> [ 1614.270585] gic_handle_irq+0x70/0x108
> [ 1614.273633] el1_irq+0xc0/0x180
> [ 1614.276526] rcu_irq_exit_irqson+0x40/0x78
> [ 1614.279704] trace_preempt_on+0x144/0x1a0
> [ 1614.282834] preempt_schedule_common+0xf8/0x1a8
> [ 1614.286126] preempt_schedule+0x38/0x40
> [ 1614.289240] __mutex_lock+0x608/0x8e8
> [ 1614.292302] mutex_lock_nested+0x3c/0x58
> [ 1614.295450] static_key_enable_cpuslocked+0x7c/0xf8
> [ 1614.298828] static_key_enable+0x2c/0x40
> [ 1614.301961] tracepoint_probe_register_prio+0x284/0x3a0
> [ 1614.305464] tracepoint_probe_register+0x40/0x58
> [ 1614.308776] trace_event_reg+0xe8/0x150
> [ 1614.311852] __ftrace_event_enable_disable+0x2e8/0x608
> [ 1614.315351] __ftrace_set_clr_event_nolock+0x160/0x1d8
> [ 1614.318809] __ftrace_set_clr_event+0x60/0x90
> [ 1614.322061] event_trace_self_tests+0x64/0x12c
> [ 1614.325335] event_trace_self_tests_init+0x88/0xa8
> [ 1614.328758] do_one_initcall+0xa4/0x500
> [ 1614.331860] kernel_init_freeable+0x344/0x3c4
> [ 1614.335110] kernel_init+0x20/0x16c
> [ 1614.338102] ret_from_fork+0x10/0x34
> [ 1614.341057] irq event stamp: 3206302
> [ 1614.344123] hardirqs last enabled at (3206301): [<ffffd9425238da04>] rcu_irq_exit_irqson+0x64/0x78
> [ 1614.348697] hardirqs last disabled at (3206302): [<ffffd942522123c0>] el1_irq+0x80/0x180
> [ 1614.353013] softirqs last enabled at (3204216): [<ffffd94252210b80>] __do_softirq+0x630/0x6b4
> [ 1614.357509] softirqs last disabled at (3204191): [<ffffd942522c623c>] irq_exit+0x1cc/0x1e0
> [ 1614.361737]
> [ 1614.361737] other info that might help us debug this:
> [ 1614.365566] Possible unsafe locking scenario:
> [ 1614.365566]
> [ 1614.369128] CPU0
> [ 1614.371747] ----
> [ 1614.374282] lock(rcu_node_0);
> [ 1614.378818] <Interrupt>
> [ 1614.381394] lock(rcu_node_0);
> [ 1614.385997]
> [ 1614.385997] *** DEADLOCK ***
> [ 1614.385997]
> [ 1614.389613] 5 locks held by swapper/0/1:
> [ 1614.392655] #0: ffffd9425480e940 (event_mutex){+.+.}-{3:3}, at: __ftrace_set_clr_event+0x48/0x90
> [ 1614.401701] #1: ffffd9425480a530 (tracepoints_mutex){+.+.}-{3:3}, at: tracepoint_probe_register_prio+0x48/0x3a0
> [ 1614.410973] #2: ffffd9425476abf0 (cpu_hotplug_lock){++++}-{0:0}, at: static_key_enable+0x24/0x40
> [ 1614.419858] #3: ffffd94254816348 (jump_label_mutex){+.+.}-{3:3}, at: static_key_enable_cpuslocked+0x7c/0xf8
> [ 1614.429049] #4: ffffd942547f47d8 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x7c0/0x17a0
> [ 1614.438029]
> [ 1614.438029] stack backtrace:
> [ 1614.441436] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.10.0-rc4-next-20201119-00004-g77838ee21ff6-dirty #21
> [ 1614.446149] Hardware name: linux,dummy-virt (DT)
> [ 1614.449621] Call trace:
> [ 1614.452337] dump_backtrace+0x0/0x240
> [ 1614.455372] show_stack+0x34/0x88
> [ 1614.458306] dump_stack+0x140/0x1bc
> [ 1614.461258] print_usage_bug+0x2a0/0x2f0
> [ 1614.464399] mark_lock.part.0+0x438/0x4e8
> [ 1614.467528] mark_held_locks+0x54/0x90
> [ 1614.470576] lockdep_hardirqs_on_prepare+0xe0/0x290
> [ 1614.473935] trace_hardirqs_on+0x90/0x370
> [ 1614.477045] el1_irq+0xdc/0x180
> [ 1614.479934] rcu_irq_exit_irqson+0x40/0x78
> [ 1614.483093] trace_preempt_on+0x144/0x1a0
> [ 1614.486211] preempt_schedule_common+0xf8/0x1a8
> [ 1614.489479] preempt_schedule+0x38/0x40
> [ 1614.492544] __mutex_lock+0x608/0x8e8
>
>
> The above has:
>
> preempt_schedule_common() {
> trace_preempt_on() {
> <interrupt>
> el1_irq:
> handle_arch_irq {
> irq_enter();
> [..]
> irq_exit();
> }
> bl trace_hardirqs_on
>
>
> I wonder if the lockdep logic got confused on ARM64 by the rework done to
> lockdep and tracing with respect to irq entry / exit.
>

I'm also staring at this problem and another thing caused my attention
is that there is a line like the following after the lockdep splat:

[...] BUG: scheduling while atomic ...

, which means preemption count has some inconsistency too.

Given this, a possible case cause this is that we got preempted inside a
rcu_node lock critical section (I know, this is quite impossible, but
preemption count and lockdep data are maintained quite separately, so
it's unlikely they are broken at the same time...)

Will continue to look into this.

Regards,
Boqun

> Or maybe there's an rcu_node leak lock that happened somewhere?
>
> -- Steve