Date:   Tue, 16 Jun 2020 19:17:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, frederic@...nel.org
Subject: Re: [PATCH 0/6] sched: TTWU, IPI, and assorted stuff

On Tue, Jun 16, 2020 at 07:04:10PM +0200, Peter Zijlstra wrote:
> [19324.742887] BUG: kernel NULL pointer dereference, address: 0000000000000150
> [19324.744075] #PF: supervisor read access in kernel mode
> [19324.744919] #PF: error_code(0x0000) - not-present page
> [19324.745786] PGD 0 P4D 0
> [19324.746215] Oops: 0000 [#1] PREEMPT SMP PTI
> [19324.746948] CPU: 10 PID: 76 Comm: ksoftirqd/10 Tainted: G        W         5.8.0-rc1+ #8
> [19324.748080] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-1 04/01/2014
> [19324.749372] RIP: 0010:check_preempt_wakeup+0xad/0x1a0
> [19324.750218] Code: d0 39 d0 7d 2c 83 ea 01 48 8b 9b 48 01 00 00 39 d0 75 f2 48 39 bb 50 01 00 00 74 1e 48 8b ad 48 01 00 00 48 8b 9b 48 01 00 00 <48> 8b bd 50 01 00 00 48 39 bb 50 01 00 00 75 e2 48 85 ff 74 dd e8
> [19324.753364] RSP: 0000:ffffb3cb40320f50 EFLAGS: 00010087
> [19324.754255] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffffb400bce0
> [19324.755465] RDX: 0000000000000000 RSI: ffff93c1dbed5b00 RDI: ffff93c1df4a8380
> [19324.756682] RBP: 0000000000000000 R08: 0000000000000000 R09: ffff93c1df2e83b0
> [19324.757848] R10: 0000000000000001 R11: 0000000000000335 R12: 0000000000000001
> [19324.758453] smpboot: CPU 11 is now offline
> [19324.759099] R13: ffff93c1dcf48000 R14: ffff93c1df4a8340 R15: ffff93c1df4a8340
> [19324.761167] FS:  0000000000000000(0000) GS:ffff93c1df480000(0000) knlGS:0000000000000000
> [19324.762559] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [19324.763527] CR2: 0000000000000150 CR3: 000000001e40a000 CR4: 00000000000006e0
> [19324.764726] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [19324.765929] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [19324.767100] Call Trace:
> [19324.767516]  <IRQ>
> [19324.767875]  check_preempt_curr+0x62/0x90
> [19324.768586]  ttwu_do_wakeup.constprop.0+0xf/0x100
> [19324.769407]  sched_ttwu_pending+0xa9/0xe0
> [19324.770077]  __sysvec_call_function_single+0x28/0xe0
> [19324.770926]  asm_call_on_stack+0x12/0x20
> [19324.771594]  </IRQ>
> [19324.771951]  sysvec_call_function_single+0x94/0xd0
> [19324.772596]  asm_sysvec_call_function_single+0x12/0x20
> [19324.773254] RIP: 0010:_raw_spin_unlock_irqrestore+0x5/0x30
> [19324.774169] Code: e4 49 ff c3 90 c6 07 00 bf 01 00 00 00 e8 23 2d 53 ff 65 8b 05 cc 32 4b 4c 85 c0 74 01 c3 e8 b9 e4 49 ff c3 90 c6 07 00 56 9d <bf> 01 00 00 00 e8 01 2d 53 ff 65 8b 05 aa 32 4b 4c 85 c0 74 01 c3
> [19324.777267] RSP: 0000:ffffb3cb4030bd58 EFLAGS: 00000287
> [19324.777956] RAX: 0000000000000001 RBX: ffff93c1dbed5b00 RCX: ffff93c1dcd63400
> [19324.779015] RDX: 0000000000000000 RSI: 0000000000000287 RDI: ffff93c1dbed6284
> [19324.780067] RBP: 000000000000000a R08: 00001193646cd91c R09: ffff93c1df49c008
> [19324.781192] R10: ffffb3cb4030bdf8 R11: 000000000000032e R12: 0000000000000000
> [19324.782386] R13: 0000000000000287 R14: ffff93c1dbed6284 R15: ffff93c1df2e8340
> [19324.783565]  try_to_wake_up+0x232/0x530
> [19324.784057]  ? trace_raw_output_hrtimer_start+0x70/0x70
> [19324.784977]  call_timer_fn+0x28/0x150
> [19324.785606]  ? trace_raw_output_hrtimer_start+0x70/0x70
> [19324.786486]  run_timer_softirq+0x182/0x250
> [19324.787191]  ? set_next_entity+0x8b/0x1a0
> [19324.787867]  ? _raw_spin_unlock_irq+0xe/0x20
> [19324.788597]  ? finish_task_switch+0x7b/0x230
> [19324.789338]  __do_softirq+0xfc/0x32b
> [19324.789961]  ? smpboot_register_percpu_thread+0xd0/0xd0
> [19324.790904]  run_ksoftirqd+0x21/0x30
> [19324.791510]  smpboot_thread_fn+0x195/0x230
> [19324.792203]  kthread+0x13d/0x160
> [19324.792731]  ? kthread_create_worker_on_cpu+0x60/0x60
> [19324.793576]  ret_from_fork+0x22/0x30
> [19324.794186] Modules linked in:
> [19324.794729] CR2: 0000000000000150

OK, so above we run the IPI handler, while below we trigger the IPI
handler on the local CPU (something that'll make, for instance Power,
explode but -just-works- on x86). Both on CPU 10.

The question is, what actual order did this happen in, I expect the WARN
happened first, and then the NULL deref. Seeing how a NULL deref is
fatal and can't very well continue to produce a WARN. I expect they got
inverted by the magic gunk that is printk.

> [19324.795303] ------------[ cut here ]------------
> [19324.795304] WARNING: CPU: 10 PID: 76 at kernel/smp.c:138 __smp_call_single_queue+0x40/0x50
> [19324.795305] Modules linked in:
> [19324.795306] CPU: 10 PID: 76 Comm: ksoftirqd/10 Not tainted 5.8.0-rc1+ #8
> [19324.795307] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-1 04/01/2014
> [19324.795307] RIP: 0010:__smp_call_single_queue+0x40/0x50
> [19324.795308] Code: c2 40 91 02 00 4c 89 e6 4c 89 e7 48 03 14 c5 e0 56 2d b4 e8 b2 3a 2f 00 84 c0 75 04 5d 41 5c c3 89 ef 5d 41 5c e9 40 af f9 ff <0f> 0b eb cd 66 66 2e 0f 1f 84 00 00 00 00 00 90 41 54 49 89 f4 55
> [19324.795309] RSP: 0000:ffffb3cb4030bd18 EFLAGS: 00010046
> [19324.795310] RAX: 000000000000000a RBX: 0000000000000000 RCX: 00000000ffffffff
> [19324.795310] RDX: 00000000000090aa RSI: ffffffffb420bc3f RDI: ffffffffb4232e3e
> [19324.795311] RBP: 000000000000000a R08: 00001193646cd91c R09: ffff93c1df49c008
> [19324.795312] R10: ffffb3cb4030bdf8 R11: 000000000000032e R12: ffff93c1dbed5b30
> [19324.795312] R13: ffff93c1df4a8340 R14: 000000000000000a R15: ffff93c1df2e8340
> [19324.795313] FS:  0000000000000000(0000) GS:ffff93c1df480000(0000) knlGS:0000000000000000
> [19324.795313] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [19324.795314] CR2: 00000000ffffffff CR3: 000000001e40a000 CR4: 00000000000006e0
> [19324.795315] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [19324.795315] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [19324.795316] Call Trace:
> [19324.795316]  ttwu_queue_wakelist+0xa4/0xc0
> [19324.795316]  try_to_wake_up+0x432/0x530

This is indeed WF_ON_CPU... it had to be, but how ?!

> [19324.795317]  ? trace_raw_output_hrtimer_start+0x70/0x70
> [19324.795317]  call_timer_fn+0x28/0x150
> [19324.795318]  ? trace_raw_output_hrtimer_start+0x70/0x70
> [19324.795318]  run_timer_softirq+0x182/0x250
> [19324.795319]  ? set_next_entity+0x8b/0x1a0
> [19324.795319]  ? _raw_spin_unlock_irq+0xe/0x20
> [19324.795319]  ? finish_task_switch+0x7b/0x230
> [19324.795320]  __do_softirq+0xfc/0x32b
> [19324.795320]  ? smpboot_register_percpu_thread+0xd0/0xd0
> [19324.795321]  run_ksoftirqd+0x21/0x30
> [19324.795321]  smpboot_thread_fn+0x195/0x230
> [19324.795321]  kthread+0x13d/0x160
> [19324.795322]  ? kthread_create_worker_on_cpu+0x60/0x60
> [19324.795322]  ret_from_fork+0x22/0x30
> [19324.795323] ---[ end trace 851fe1f1f7a85d8b ]---


> [19324.828475] ---[ end trace 851fe1f1f7a85d8c ]---
> [19324.829250] RIP: 0010:check_preempt_wakeup+0xad/0x1a0
> [19324.830107] Code: d0 39 d0 7d 2c 83 ea 01 48 8b 9b 48 01 00 00 39 d0 75 f2 48 39 bb 50 01 00 00 74 1e 48 8b ad 48 01 00 00 48 8b 9b 48 01 00 00 <48> 8b bd 50 01 00 00 48 39 bb 50 01 00 00 75 e2 48 85 ff 74 dd e8
> [19324.833208] RSP: 0000:ffffb3cb40320f50 EFLAGS: 00010087
> [19324.834098] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffffb400bce0
> [19324.835272] RDX: 0000000000000000 RSI: ffff93c1dbed5b00 RDI: ffff93c1df4a8380
> [19324.836466] RBP: 0000000000000000 R08: 0000000000000000 R09: ffff93c1df2e83b0
> [19324.837669] R10: 0000000000000001 R11: 0000000000000335 R12: 0000000000000001
> [19324.838867] R13: ffff93c1dcf48000 R14: ffff93c1df4a8340 R15: ffff93c1df4a8340
> [19324.840019] FS:  0000000000000000(0000) GS:ffff93c1df480000(0000) knlGS:0000000000000000
> [19324.841316] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [19324.842242] CR2: 0000000000000150 CR3: 000000001e40a000 CR4: 00000000000006e0
> [19324.843406] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [19324.844568] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [19324.845710] Kernel panic - not syncing: Fatal exception in interrupt
> [19324.846998] Kernel Offset: 0x32000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
> [19324.848713] ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---

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