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SubjectNULL pointer dereference in pick_next_task_fair
Hi all,

The following issue has been observed with 5.4-rc*-based android
kernels both on an x86 VM and native arm64:

---8<---
[ 222.515957] BUG: kernel NULL pointer dereference, address: 0000000000000040
[ 222.518871] #PF: supervisor read access in kernel mode
[ 222.518871] #PF: error_code(0x0000) - not-present page
[ 222.518871] PGD 800000007cdf7067 P4D 800000007cdf7067 PUD 7c868067 PMD 0
[ 222.518871] Oops: 0000 [#1] PREEMPT SMP PTI
[ 222.518871] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 5.4.0-rc4-mainline-g6a9b34166c05 #1
[ 222.518871] Hardware name: ChromiumOS crosvm, BIOS 0
[ 222.518871] RIP: 0010:set_next_entity+0xf/0xf0
[ 222.518871] Code: 42 0b e5 85 31 c0 e8 90 1a fc ff 0f 0b eb 9b 66 90 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 41 57 41 56 53 48 89 f3 49 89 fe <83> 7e 40 00 74 3d 4c 89 f7 48 89 de e8 80 f9 ff ff 4c 8d 7b 18 4d
[ 222.518871] RSP: 0018:ffffb95040073de8 EFLAGS: 00010046
[ 222.518871] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff85ec0c30
[ 222.518871] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff9ec44c728500
[ 222.518871] RBP: ffffb95040073e00 R08: 0000000000000000 R09: 0000000000000000
[ 222.518871] R10: 0000000000000000 R11: ffffffff84ef0a30 R12: 0000000000000000
[ 222.518871] R13: 0000000000000000 R14: ffff9ec44c728500 R15: ffffb95040073e60
[ 222.518871] FS: 0000000000000000(0000) GS:ffff9ec44c700000(0000) knlGS:0000000000000000
[ 222.518871] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 222.518871] CR2: 0000000000000040 CR3: 000000007c87a002 CR4: 00000000003606e0
[ 222.518871] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 222.518871] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 222.518871] Call Trace:
[ 222.518871] pick_next_task_fair+0xe8/0x410
[ 222.518871] ? rcu_note_context_switch+0x3ef/0x520
[ 222.518871] ? finish_task_switch+0x10d/0x250
[ 222.518871] __schedule+0x1f8/0x6e0
[ 222.518871] schedule_idle+0x27/0x40
[ 222.518871] do_idle+0x21c/0x240
[ 222.518871] cpu_startup_entry+0x25/0x30
[ 222.518871] start_secondary+0x18d/0x1b0
[ 222.518871] secondary_startup_64+0xa4/0xb0
[ 222.518871] Modules linked in: vmw_vsock_virtio_transport vmw_vsock_virtio_transport_common vsock_diag vsock can_raw can snd_intel8x0 snd_ac97_codec ac97_bus virtio_input virtio_pci virtio_mmio virtio_crypto mmc_block mmc_core dummy_cpufreq rtc_test dummy_hcd can_dev vcan virtio_net net_failover failover virt_wifi virtio_blk virtio_gpu ttm virtio_rng virtio_ring virtio 8250_of crypto_engine
[ 222.518871] CR2: 0000000000000040
[ 222.518871] ---[ end trace ae741809965b22f2 ]---
[ 222.518871] RIP: 0010:set_next_entity+0xf/0xf0
[ 222.518871] Code: 42 0b e5 85 31 c0 e8 90 1a fc ff 0f 0b eb 9b 66 90 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 41 57 41 56 53 48 89 f3 49 89 fe <83> 7e 40 00 74 3d 4c 89 f7 48 89 de e8 80 f9 ff ff 4c 8d 7b 18 4d
[ 222.518871] RSP: 0018:ffffb95040073de8 EFLAGS: 00010046
[ 222.518871] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff85ec0c30
[ 222.518871] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff9ec44c728500
[ 222.518871] RBP: ffffb95040073e00 R08: 0000000000000000 R09: 0000000000000000
[ 222.518871] R10: 0000000000000000 R11: ffffffff84ef0a30 R12: 0000000000000000
[ 222.518871] R13: 0000000000000000 R14: ffff9ec44c728500 R15: ffffb95040073e60
[ 222.518871] FS: 0000000000000000(0000) GS:ffff9ec44c700000(0000) knlGS:0000000000000000
[ 222.518871] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 222.518871] CR2: 0000000000000040 CR3: 000000007c87a002 CR4: 00000000003606e0
[ 222.518871] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 222.518871] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 222.518871] Kernel panic - not syncing: Fatal exception
[ 222.518871] Kernel Offset: 0x3e00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
--->8---

The issue is very transient and relatively hard to reproduce.

After digging a bit, the offending commit seems to be:

67692435c411 ("sched: Rework pick_next_task() slow-path")

By 'offending' I mean that reverting it makes the issue go away. The
issue comes from the fact that pick_next_entity() returns a NULL se in
the 'simple' path of pick_next_task_fair(), which causes obvious
problems in the subsequent call to set_next_entity().

I'll dig more, but if anybody understands the issue in the meatime feel
free to send me a patch to try out :)

Thanks,
Quentin

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