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Date:   Wed, 16 Dec 2020 17:29:39 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: RCU stall leading to deadlock warning

Hi Paul

We hit the below splat a couple of days ago in our testing. Sadly I can't
reproduce it. And it was on android-mainline branch..

It's the deadlock message that bothers me. I can't see how we could have ended
there. We detect a stall and when trying to dump the stack LOCKDEP spits the
warning.

Maybe should take this report with a pinch of salt since it wasn't on mainline.
I just thought it might be something worth sharing in case you can actually
spot something obvious that I can't see. If I got more info or a reproducer
I will share them.

The failure was triggered twice on that day running 2 different tests.

[  310.073379] LTP: starting leapsec01
[  345.070123] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
[  345.076717] rcu:     0-...!: (1 ticks this GP) idle=798/0/0x0 softirq=19187/19187 fqs=0  (false positive?)
[  345.087533]    (detected by 2, t=6502 jiffies, g=57249, q=184)
[  345.093284]
[  345.094797] ============================================
[  345.100139] WARNING: possible recursive locking detected
[  345.105485] 5.10.0-rc7-02296-g3f43bd6f2c3b-ab89 #1 Not tainted
[  345.111349] --------------------------------------------
[  345.116693] swapper/2/0 is trying to acquire lock:
[  345.121515] ffffa00013b50c58 (rcu_node_0){-.-.}-{2:2}, at: rcu_dump_cpu_stacks+0x7c/0x14c
[  345.129813]
[  345.129813] but task is already holding lock:
[  345.135678] ffffa00013b50c58 (rcu_node_0){-.-.}-{2:2}, at: rcu_sched_clock_irq+0x68c/0x11c0
[  345.144143]
[  345.144143] other info that might help us debug this:
[  345.150702]  Possible unsafe locking scenario:
[  345.150702]
[  345.156651]        CPU0
[  345.159119]        ----
[  345.161585]   lock(rcu_node_0);
[  345.164779]   lock(rcu_node_0);
[  345.167973]
[  345.167973]  *** DEADLOCK ***
[  345.167973]
[  345.173923]  May be due to missing lock nesting notation
[  345.173923]
[  345.180746] 1 lock held by swapper/2/0:
[  345.184607]  #0: ffffa00013b50c58 (rcu_node_0){-.-.}-{2:2}, at: rcu_sched_clock_irq+0x68c/0x11c0
[  345.193517]
[  345.193517] stack backtrace:
[  345.197910] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 5.10.0-rc7-02296-g3f43bd6f2c3b-ab89 #1
[  345.206389] Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno Development Platform, BIOS EDK II Oct 19 2019
[  345.217215] Call trace:
[  345.219691]  dump_backtrace+0x0/0x2b8
[  345.223383]  show_stack+0x18/0x68
[  345.226731]  dump_stack+0x110/0x188
[  345.230255]  __lock_acquire+0x23f0/0x2410
[  345.234300]  lock_acquire+0x3b8/0x730
[  345.237997]  _raw_spin_lock_irqsave+0x80/0x168
[  345.242476]  rcu_dump_cpu_stacks+0x7c/0x14c
[  345.246693]  rcu_sched_clock_irq+0xfd4/0x11c0
[  345.251087]  update_process_times+0x84/0xe0
[  345.255306]  tick_sched_handle.isra.0+0x68/0x98
[  345.259871]  tick_sched_timer+0x60/0xd8
[  345.263742]  __hrtimer_run_queues+0x534/0x9e0
[  345.268134]  hrtimer_interrupt+0x1a8/0x398
[  345.272264]  tick_receive_broadcast+0x60/0x88
[  345.276657]  ipi_handler+0x250/0x4b8
[  345.280270]  handle_percpu_devid_fasteoi_ipi+0x138/0x4f0
[  345.285619]  generic_handle_irq+0x4c/0x68
[  345.289661]  __handle_domain_irq+0x9c/0x118
[  345.293880]  gic_handle_irq+0xdc/0x118
[  345.297661]  el1_irq+0xc8/0x180
[  345.300835]  cpuidle_enter_state+0x16c/0x810
[  345.305139]  cpuidle_enter+0x4c/0x78
[  345.308749]  call_cpuidle+0x44/0x88
[  345.312271]  do_idle+0x2d4/0x338
[  345.315532]  cpu_startup_entry+0x24/0x68
[  345.319491]  secondary_start_kernel+0x1d4/0x2d8

Thanks

--
Qais Yousef

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