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Message-ID: <20201112181254.GA3113918@elver.google.com>
Date: Thu, 12 Nov 2020 19:12:54 +0100
From: Marco Elver <elver@...gle.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Anders Roxell <anders.roxell@...aro.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Alexander Potapenko <glider@...gle.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Jann Horn <jannh@...gle.com>,
Mark Rutland <mark.rutland@....com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>,
kasan-dev <kasan-dev@...glegroups.com>, rcu@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] kfence: Avoid stalling work queue task without
allocations
On Thu, Nov 12, 2020 at 09:54AM -0800, Paul E. McKenney wrote:
> On Thu, Nov 12, 2020 at 05:14:39PM +0100, Marco Elver wrote:
> > On Thu, Nov 12, 2020 at 01:49PM +0100, Marco Elver wrote:
> > > On Thu, 12 Nov 2020 at 01:11, Paul E. McKenney <paulmck@...nel.org> wrote:
> > [...]
> > > > > This assert didn't fire yet, I just get more of the below. I'll keep
> > > > > rerunning, but am not too hopeful...
> > > >
> > > > Is bisection a possibility?
> > >
> > > I've been running a bisection for past ~12h, and am making slow
> > > progress. It might be another 12h, but I think it'll get there.
> >
> > Bisection gave me this:
> >
> > | git bisect start
> > | # bad: [c07b306d7fa5680777e2132662d2e6c19fb53579] kfence: Avoid stalling work queue task without allocations
> > | git bisect bad c07b306d7fa5680777e2132662d2e6c19fb53579
> > | # good: [3cea11cd5e3b00d91caf0b4730194039b45c5891] Linux 5.10-rc2
> > | git bisect good 27598e7e73260ed0b2917eb02d4a515ebb578313
> > | # good: [3e5acbea719e66ef3be64fe74c99cc905ca697dc] Merge remote-tracking branch 'wireless-drivers-next/master' into master
> > | git bisect good 3e5acbea719e66ef3be64fe74c99cc905ca697dc
> > | # good: [491a5a9a2fea28353d99621b8abb83b6928b4e36] Merge remote-tracking branch 'sound-asoc/for-next' into master
> > | git bisect good 491a5a9a2fea28353d99621b8abb83b6928b4e36
> > | # bad: [502f8643d6e21c7e370a0b75131130cc51609055] Merge remote-tracking branch 'phy-next/next' into master
> > | git bisect bad 502f8643d6e21c7e370a0b75131130cc51609055
> > | # good: [6693cb1fa5ea7b91ec00f9404776a095713face5] Merge remote-tracking branch 'tip/auto-latest' into master
> > | git bisect good 6693cb1fa5ea7b91ec00f9404776a095713face5
> > | # bad: [b790e3afead9357195b6d1e1b6cd9b3521503ad2] Merge branch 'tglx-pc.2020.10.30a' into HEAD
> > | git bisect bad b790e3afead9357195b6d1e1b6cd9b3521503ad2
> > | # bad: [765b512bb3d639bfad7dd43c288ee085236c7267] Merge branches 'cpuinfo.2020.11.06a', 'doc.2020.11.06a', 'fixes.2020.11.02a', 'lockdep.2020.11.02a', 'tasks.2020.11.06a' and 'torture.2020.11.06a' into HEAD
> > | git bisect bad 765b512bb3d639bfad7dd43c288ee085236c7267
> > | # good: [01f9e708d9eae6335ae9ff25ab09893c20727a55] tools/rcutorture: Fix BUG parsing of console.log
>
> So torture.2020.11.06a is OK.
>
> > | git bisect good 01f9e708d9eae6335ae9ff25ab09893c20727a55
> > | # good: [1be6ab91e2db157faedb7f16ab0636a80745a073] srcu: Take early exit on memory-allocation failure
>
> As is fixes.2020.11.02a.
>
> > | git bisect good 1be6ab91e2db157faedb7f16ab0636a80745a073
> > | # good: [65e9eb1ccfe56b41a0d8bfec651ea014968413cb] rcu: Prevent RCU_LOCKDEP_WARN() from swallowing the condition
>
> And lockdep.2020.11.02a.
>
> > | git bisect good 65e9eb1ccfe56b41a0d8bfec651ea014968413cb
> > | # good: [c386e29d43728778ddd642fa73cc582bee684171] docs/rcu: Update the call_rcu() API
>
> And doc.2020.11.06a.
>
> > | git bisect good c386e29d43728778ddd642fa73cc582bee684171
> > | # good: [27c0f1448389baf7f309b69e62d4b531c9395e88] rcutorture: Make grace-period kthread report match RCU flavor being tested
>
> And the first three commits of tasks.2020.11.06a.
>
> > | git bisect good 27c0f1448389baf7f309b69e62d4b531c9395e88
> > | # good: [3fcd6a230fa7d03bffcb831a81b40435c146c12b] x86/cpu: Avoid cpuinfo-induced IPIing of idle CPUs
>
> And cpuinfo.2020.11.06a.
>
> > | git bisect good 3fcd6a230fa7d03bffcb831a81b40435c146c12b
> > | # good: [75dc2da5ecd65bdcbfc4d59b9d9b7342c61fe374] rcu-tasks: Make the units of ->init_fract be jiffies
>
> And the remaining commit of tasks.2020.11.06a.
>
> > | git bisect good 75dc2da5ecd65bdcbfc4d59b9d9b7342c61fe374
> > | # first bad commit: [765b512bb3d639bfad7dd43c288ee085236c7267] Merge branches 'cpuinfo.2020.11.06a', 'doc.2020.11.06a', 'fixes.2020.11.02a', 'lockdep.2020.11.02a', 'tasks.2020.11.06a' and 'torture.2020.11.06a' into HEAD
> >
> > This doesn't look very satisfying, given it's the merge commit. :-/
>
> So each individual branch is just fine, but the merge of them is not. Fun.
>
> These have been passing quite a bit of rcutorture over here, including
> preemptible kernels running !SMP, but admittedly on x86 rather than ARMv8.
Note that this is ARMv8 on QEMU on an x86 host i.e. emulated. And it's
really slow as a result. Together with a bunch of debug tools including
lockdep.
> One approach would be to binary-search the combinations of merges.
> Except that there are six of them, so there are 64 combinations, of
> which you have tested only 8 thus far (none, one each, and all).
>
> But are you sure that the bisection points labeled "good" really are good?
> For example, what is the distribution of first failure times in the
> points labeled "bad" vs. the runtime used to make a "good" determination?
> Alternatively, just try a longer run on each of the commits feeding into
> the merge point.
Yeah, I'm having doubts, and this might be even more non-deterministic
that I thought and some 'good' could maybe be 'bad' if I had re-run
them? I don't know. One thing I can try is to make sure I run it more
than once, but I'm definitely not doing that manually, so let me try and
script something so I don't have to hand-hold the bisection overnight.
:-)
> > > > Failing that, please see the updated patch below. This adds a few more
> > > > calls to lockdep_assert_irqs_disabled(), but perhaps more helpfully dumps
> > > > the current stack of the CPU that the RCU grace-period kthread wants to
> > > > run on in the case where this kthread has been starved of CPU.
> > >
> > > Thanks, I will apply that after the bisection runs.
> >
> > Here's a new log with it applied:
>
> Even more strangeness! ;-)
>
> > | [ 118.480959] Key type dns_resolver registered
> > | [ 118.487752] registered taskstats version 1
> > | [ 118.489798] Running tests on all trace events:
> > | [ 118.490164] Testing all events: OK
> > | [ 173.304186] Running tests again, along with the function tracer
> > | [ 173.320155] Running tests on all trace events:
> > | [ 173.331638] Testing all events:
> > | [ 173.485044] hrtimer: interrupt took 14340976 ns
>
> Fourteen milliseconds, so annoying from a real-time perspective, but
> unlikely to be the cause of this.
>
> Was the system responsive at this point, between three and ten minutes
> after boot? Similar question for the other gaps in the dmesg log.
> The reason for the question is that workqueue's reported stall times
> don't span these intervals.
The system is so slow at this point that I can't get much out of it
either way, other than waiting and seeing if it proceeds...
> > | [ 334.160218] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 15s!
>
> It might be instructive to cause this code to provoke a backtrace.
> I suggest adding something like "trigger_single_cpu_backtrace(cpu)"
> in kernel/workqueue.c's function named wq_watchdog_timer_fn()
> somewhere within its "if" statement that is preceded with the "did we
> stall?" comment. Or just search for "BUG: workqueue lockup - pool"
> within kernel/workqueue.c.
>
> > | [ 334.259490] Showing busy workqueues and worker pools:
> > | [ 334.265398] workqueue events: flags=0x0
> > | [ 334.289070] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> > | [ 334.300659] pending: vmstat_shepherd
> > | [ 453.541827] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 10s!
> > | [ 453.655731] BUG: workqueue lockup - pool cpus=0 flags=0x4 nice=0 stuck for 10s!
> > | [ 453.759839] Showing busy workqueues and worker pools:
> > | [ 453.784294] workqueue events: flags=0x0
> > | [ 453.812207] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> > | [ 453.822108] pending: vmstat_shepherd
> > | [ 453.839855] workqueue events_power_efficient: flags=0x82
> > | [ 453.865152] pwq 2: cpus=0 flags=0x4 nice=0 active=2/256 refcnt=4
> > | [ 453.874553] pending: neigh_periodic_work, do_cache_clean
> > | [ 481.424362] BUG: workqueue lockup - pool cpus=0 flags=0x4 nice=0 stuck for 10s!
> > | [ 481.508136] Showing busy workqueues and worker pools:
> > | [ 481.524265] workqueue events: flags=0x0
> > | [ 481.550480] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> > | [ 481.560690] pending: vmstat_shepherd
> > | [ 481.571255] workqueue events_power_efficient: flags=0x82
> > | [ 481.592515] pwq 2: cpus=0 flags=0x4 nice=0 active=1/256 refcnt=3
> > | [ 481.601153] pending: neigh_periodic_work
> > | [ 532.108407] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 10s!
> > | [ 532.203476] Showing busy workqueues and worker pools:
> > | [ 532.215930] workqueue events: flags=0x0
> > | [ 532.244203] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> > | [ 532.254428] pending: vmstat_shepherd
> > | [ 739.567892] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 19s!
> > | [ 739.656419] Showing busy workqueues and worker pools:
> > | [ 739.699514] workqueue events: flags=0x0
> > | [ 739.705111] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> > | [ 739.715393] pending: vmstat_shepherd
> > | [ 739.733403] workqueue events_power_efficient: flags=0x82
> > | [ 739.739433] pwq 2: cpus=0 flags=0x4 nice=0 active=2/256 refcnt=4
> > | [ 739.748156] pending: check_lifetime, neigh_periodic_work
> > | [ 811.578165] BUG: workqueue lockup - pool cpus=0 flags=0x5 nice=0 stuck for 14s!
> > | [ 811.602913] Showing busy workqueues and worker pools:
> > | [ 811.620424] workqueue events: flags=0x0
> > | [ 811.652479] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> > | [ 811.662686] pending: vmstat_shepherd
> > | [ 811.683811] workqueue events_power_efficient: flags=0x82
> > | [ 811.716123] pwq 2: cpus=0 flags=0x5 nice=0 active=1/256 refcnt=3
> > | [ 811.724857] pending: neigh_periodic_work
> > | [ 811.749989] pool 2: cpus=0 flags=0x5 nice=0 hung=14s workers=2 manager: 61 idle: 7
> > | [ 822.456290] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 11s!
> > | [ 822.600359] BUG: workqueue lockup - pool cpus=0 flags=0x5 nice=0 stuck for 25s!
> > | [ 822.675814] Showing busy workqueues and worker pools:
> > | [ 822.720098] workqueue events: flags=0x0
> > | [ 822.747304] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> > | [ 822.757174] pending: vmstat_shepherd
> > | [ 822.768047] workqueue events_power_efficient: flags=0x82
> > | [ 822.799954] pwq 2: cpus=0 flags=0x5 nice=0 active=1/256 refcnt=3
> > | [ 822.808488] pending: neigh_periodic_work
> > | [ 822.831900] pool 2: cpus=0 flags=0x5 nice=0 hung=25s workers=2 manager: 61 idle: 7
> > | [ 834.116239] BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 22s!
> > | [ 834.246557] BUG: workqueue lockup - pool cpus=0 flags=0x5 nice=0 stuck for 37s!
> > | [ 834.271069] Showing busy workqueues and worker pools:
> > | [ 834.276687] workqueue events: flags=0x0
> > | [ 834.296267] pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> > | [ 834.306148] pending: vmstat_shepherd
> > | [ 834.324273] workqueue events_power_efficient: flags=0x82
> > | [ 834.344433] pwq 2: cpus=0 flags=0x5 nice=0 active=2/256 refcnt=4
> > | [ 834.352891] pending: neigh_periodic_work, do_cache_clean
> > | [ 834.384530] pool 2: cpus=0 flags=0x5 nice=0 hung=37s workers=2 manager: 61 idle: 7
> > | [ 840.906940] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> > | [ 840.912685] (detected by 0, t=3752 jiffies, g=2709, q=1)
>
> CPU 0 detected the stall.
>
> > | [ 840.914587] rcu: All QSes seen, last rcu_preempt kthread activity 620 (4295099794-4295099174), jiffies_till_next_fqs=1, root ->qsmask 0x0
>
> As before, the grace period is not stalled, but instead the grace-period
> kthread is failing to detect the end of an already-ended grace period.
>
> > | [ 840.925016] rcu: rcu_preempt kthread starved for 620 jiffies! g2709 f0x2 RCU_GP_CLEANUP(7) ->state=0x0 ->cpu=0
>
> And CPU 0 is where the RCU grace-period kthread was last seen running.
>
> > | [ 840.930687] rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> > | [ 840.936056] rcu: RCU grace-period kthread stack dump:
> > | [ 840.940433] task:rcu_preempt state:R running task stack: 0 pid: 10 ppid: 2 flags:0x00000428
> > | [ 840.949160] Call trace:
> > | [ 840.952822] dump_backtrace+0x0/0x278
> > | [ 840.956816] show_stack+0x30/0x80
> > | [ 840.960643] sched_show_task+0x1a8/0x240
> > | [ 840.964684] rcu_check_gp_kthread_starvation+0x170/0x358
> > | [ 840.969113] rcu_sched_clock_irq+0x744/0xd18
> > | [ 840.973232] update_process_times+0x68/0x98
> > | [ 840.977308] tick_sched_handle.isra.16+0x54/0x80
> > | [ 840.981504] tick_sched_timer+0x64/0xd8
> > | [ 840.985500] __hrtimer_run_queues+0x2a4/0x750
> > | [ 840.989628] hrtimer_interrupt+0xf4/0x2a0
> > | [ 840.993669] arch_timer_handler_virt+0x44/0x70
> > | [ 840.997841] handle_percpu_devid_irq+0xfc/0x4d0
> > | [ 841.002043] generic_handle_irq+0x50/0x70
> > | [ 841.006098] __handle_domain_irq+0x9c/0x120
> > | [ 841.010188] gic_handle_irq+0xcc/0x108
> > | [ 841.014132] el1_irq+0xbc/0x180
> > | [ 841.017935] arch_local_irq_restore+0x4/0x8
> > | [ 841.021993] trace_preempt_on+0xf4/0x190
> > | [ 841.026016] preempt_schedule_common+0x12c/0x1b0
> > | [ 841.030193] preempt_schedule.part.88+0x20/0x28
> > | [ 841.034373] preempt_schedule+0x20/0x28
> > | [ 841.038369] _raw_spin_unlock_irq+0x80/0x90
> > | [ 841.042498] rcu_gp_kthread+0xe5c/0x19a8
> > | [ 841.046504] kthread+0x174/0x188
> > | [ 841.050320] ret_from_fork+0x10/0x18
> > | [ 841.054312] rcu: Stack dump where RCU grace-period kthread last ran:
> > | [ 841.058980] Task dump for CPU 0:
> > | [ 841.062736] task:rcu_preempt state:R running task stack: 0 pid: 10 ppid: 2 flags:0x00000428
>
> And RCU's grace-period kthread really is running on CPU 0 right now.
> It is just not making any forward progress.
>
> > | [ 841.071073] Call trace:
> > | [ 841.074662] dump_backtrace+0x0/0x278
> > | [ 841.078596] show_stack+0x30/0x80
> > | [ 841.082386] sched_show_task+0x1a8/0x240
> > | [ 841.086367] dump_cpu_task+0x48/0x58
> > | [ 841.090311] rcu_check_gp_kthread_starvation+0x214/0x358
> > | [ 841.094736] rcu_sched_clock_irq+0x744/0xd18
> > | [ 841.098852] update_process_times+0x68/0x98
> > | [ 841.102949] tick_sched_handle.isra.16+0x54/0x80
> > | [ 841.107119] tick_sched_timer+0x64/0xd8
> > | [ 841.111127] __hrtimer_run_queues+0x2a4/0x750
> > | [ 841.115264] hrtimer_interrupt+0xf4/0x2a0
> > | [ 841.119319] arch_timer_handler_virt+0x44/0x70
> > | [ 841.123525] handle_percpu_devid_irq+0xfc/0x4d0
> > | [ 841.127690] generic_handle_irq+0x50/0x70
> > | [ 841.131702] __handle_domain_irq+0x9c/0x120
> > | [ 841.135779] gic_handle_irq+0xcc/0x108
> > | [ 841.139743] el1_irq+0xbc/0x180
>
> The code above this point was detecting and printing the RCU CPU stall
> warning. The code below this point was doing what?
>
> Any chance of getting file names and line numbers for the rest of this
> stack?
I've attached a version of the log with line numbers.
> > | [ 841.143527] arch_local_irq_restore+0x4/0x8
>
> So we are just now restoring interrupts, hence our getting the
> interrupt at this point..
>
> > | [ 841.147612] trace_preempt_on+0xf4/0x190
>
> From within the trace code, which is apparently recording the fact
> that preemption is being enabled.
>
> > | [ 841.151656] preempt_schedule_common+0x12c/0x1b0
> > | [ 841.155869] preempt_schedule.part.88+0x20/0x28
> > | [ 841.160036] preempt_schedule+0x20/0x28
>
> I was not aware that releasing a raw spinlock could result in a direct
> call to preempt_schedule().
>
> > | [ 841.164051] _raw_spin_unlock_irq+0x80/0x90
> > | [ 841.168139] rcu_gp_kthread+0xe5c/0x19a8
>
> So the RCU grace-period kthread has spent many seconds attempting to
> release a lock? Am I reading this correctly? Mark Rutland, am I missing
> something here?
>
> > | [ 841.172134] kthread+0x174/0x188
> > | [ 841.175953] ret_from_fork+0x10/0x18
> > | [ 841.191371]
> > | [ 841.193648] ================================
> > | [ 841.196605] WARNING: inconsistent lock state
> > | [ 841.199764] 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #23 Not tainted
> > | [ 841.203564] --------------------------------
>
> Has lockdep recorded the fact that the lock is actually released?
> It had better, given that interrupts are now enabled.
>
> > | [ 841.206550] inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage.
> > | [ 841.210074] rcu_preempt/10 [HC0[0]:SC0[0]:HE0:SE1] takes:
> > | [ 841.213453] ffffd787e91d4358 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x4a0/0xd18
> > | [ 841.221240] {IN-HARDIRQ-W} state was registered at:
> > | [ 841.224538] __lock_acquire+0x7bc/0x15b8
> > | [ 841.227541] lock_acquire+0x244/0x498
> > | [ 841.230442] _raw_spin_lock_irqsave+0x78/0x144
> > | [ 841.233555] rcu_sched_clock_irq+0x4a0/0xd18
> > | [ 841.236621] update_process_times+0x68/0x98
> > | [ 841.239645] tick_sched_handle.isra.16+0x54/0x80
> > | [ 841.242801] tick_sched_timer+0x64/0xd8
> > | [ 841.245745] __hrtimer_run_queues+0x2a4/0x750
> > | [ 841.248842] hrtimer_interrupt+0xf4/0x2a0
> > | [ 841.251846] arch_timer_handler_virt+0x44/0x70
> > | [ 841.254976] handle_percpu_devid_irq+0xfc/0x4d0
> > | [ 841.258131] generic_handle_irq+0x50/0x70
> > | [ 841.261146] __handle_domain_irq+0x9c/0x120
> > | [ 841.264169] gic_handle_irq+0xcc/0x108
> > | [ 841.267096] el1_irq+0xbc/0x180
> > | [ 841.269844] arch_local_irq_restore+0x4/0x8
> > | [ 841.272881] trace_preempt_on+0xf4/0x190
> > | [ 841.275847] preempt_schedule_common+0x12c/0x1b0
> > | [ 841.279017] preempt_schedule.part.88+0x20/0x28
> > | [ 841.282149] preempt_schedule+0x20/0x28
> > | [ 841.285112] _raw_spin_unlock_irq+0x80/0x90
> > | [ 841.288154] rcu_gp_kthread+0xe5c/0x19a8
> > | [ 841.291175] kthread+0x174/0x188
> > | [ 841.293952] ret_from_fork+0x10/0x18
> > | [ 841.296780] irq event stamp: 39750
> > | [ 841.299604] hardirqs last enabled at (39749): [<ffffd787e6d85738>] rcu_irq_enter_irqson+0x48/0x68
> > | [ 841.303961] hardirqs last disabled at (39750): [<ffffd787e6c122bc>] el1_irq+0x7c/0x180
> > | [ 841.308042] softirqs last enabled at (36704): [<ffffd787e6c10b58>] __do_softirq+0x650/0x6a4
> > | [ 841.312250] softirqs last disabled at (36683): [<ffffd787e6cc0b80>] irq_exit+0x1a8/0x1b0
> > | [ 841.316257]
> > | [ 841.316257] other info that might help us debug this:
> > | [ 841.319834] Possible unsafe locking scenario:
> > | [ 841.319834]
> > | [ 841.323217] CPU0
> > | [ 841.325656] ----
> > | [ 841.328097] lock(rcu_node_0);
> > | [ 841.332433] <Interrupt>
> > | [ 841.334966] lock(rcu_node_0);
> > | [ 841.339379]
> > | [ 841.339379] *** DEADLOCK ***
> > | [ 841.339379]
> > | [ 841.342829] 1 lock held by rcu_preempt/10:
> > | [ 841.345794] #0: ffffd787e91d4358 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x4a0/0xd18
> > | [ 841.354415]
> > | [ 841.354415] stack backtrace:
> > | [ 841.357664] CPU: 0 PID: 10 Comm: rcu_preempt Not tainted 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #23
> > | [ 841.362249] Hardware name: linux,dummy-virt (DT)
> > | [ 841.365352] Call trace:
> > | [ 841.367862] dump_backtrace+0x0/0x278
> > | [ 841.370745] show_stack+0x30/0x80
> > | [ 841.373517] dump_stack+0x138/0x1b0
> > | [ 841.376339] print_usage_bug+0x2d8/0x2f8
> > | [ 841.379288] mark_lock.part.46+0x370/0x480
> > | [ 841.382304] mark_held_locks+0x58/0x90
> > | [ 841.385228] lockdep_hardirqs_on_prepare+0xdc/0x298
> > | [ 841.388452] trace_hardirqs_on+0x90/0x388
> > | [ 841.391434] el1_irq+0xd8/0x180
> > | [ 841.394178] arch_local_irq_restore+0x4/0x8
> > | [ 841.397186] trace_preempt_on+0xf4/0x190
> > | [ 841.400127] preempt_schedule_common+0x12c/0x1b0
> > | [ 841.403246] preempt_schedule.part.88+0x20/0x28
> > | [ 841.406347] preempt_schedule+0x20/0x28
> > | [ 841.409278] _raw_spin_unlock_irq+0x80/0x90
> > | [ 841.412290] rcu_gp_kthread+0xe5c/0x19a8
> > | [ 841.415237] kthread+0x174/0x188
> > | [ 841.418011] ret_from_fork+0x10/0x18
> > | [ 841.423450] BUG: scheduling while atomic: rcu_preempt/10/0x00000002
> > | [ 841.431367] INFO: lockdep is turned off.
> > | [ 841.439132] Modules linked in:
> > | [ 841.450608] Preemption disabled at:
> > | [ 841.452261] [<ffffd787e7fffec0>] preempt_schedule.part.88+0x20/0x28
> > | [ 841.467324] CPU: 0 PID: 10 Comm: rcu_preempt Not tainted 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #23
> > | [ 841.471926] Hardware name: linux,dummy-virt (DT)
> > | [ 841.475030] Call trace:
> > | [ 841.477581] dump_backtrace+0x0/0x278
> > | [ 841.480451] show_stack+0x30/0x80
> > | [ 841.483220] dump_stack+0x138/0x1b0
> > | [ 841.486057] __schedule_bug+0x8c/0xe8
> > | [ 841.488949] __schedule+0x7e8/0x890
> > | [ 841.491801] preempt_schedule_common+0x44/0x1b0
> > | [ 841.494927] preempt_schedule.part.88+0x20/0x28
> > | [ 841.498048] preempt_schedule+0x20/0x28
> > | [ 841.500963] _raw_spin_unlock_irq+0x80/0x90
> > | [ 841.503988] rcu_gp_kthread+0xe5c/0x19a8
> > | [ 841.506965] kthread+0x174/0x188
> > | [ 841.509732] ret_from_fork+0x10/0x18
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