kernel BUG at mm/ktsan/thr.c:129!

[dvyukov]

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kernel BUG at mm/ktsan/thr.c:129!
invalid opcode: 0000 [#3] SMP
Modules linked in:
CPU: 5 PID: 2758 Comm: sshd Tainted: G      D         4.2.0+ #396
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff880485130240 ti: ffff880480788000 task.ti: ffff880480788000
RIP: 0010:[<ffffffff8125adba>]  [<ffffffff8125adba>] kt_thr_start+0xaa/0xb0
RSP: 0018:ffff88048078be40  EFLAGS: 00010082
RAX: ffffe8ffffca0910 RBX: ffff8804d6686e70 RCX: 0000000000000001
RDX: 0000000000000001 RSI: ffffffff810c6393 RDI: 0000000000000082
RBP: 00000ac600050004 R08: ffff880480788000 R09: 0000000000000000
R10: ffff88083fcb4fe8 R11: ffff8804862b8c20 R12: 000000000000951a
R13: ffff88047e261440 R14: ffff880485130240 R15: 0000000000000040
FS:  00007f4c10bf37c0(0000) GS:ffff88083fca0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000000000 CR3: 0000000485ed4000 CR4: 00000000000006e0
Stack:
 ffff8804d6686e70 0000000000000286 ffff88048594d0c0 ffffffff812531e6
 ffff880485130240 ffff88083fcb4f80 ffff88048078bec0 ffffffff810c6393
 ffff8804862165c0 ffff88083fcb4f80 ffff88047e13a540 ffff88083fcb4f80
Call Trace:
 [<ffffffff812531e6>] ? ktsan_thr_start+0x66/0x90 mm/ktsan/ktsan.c:312 (discriminator 7)
 [<ffffffff810c6393>] ? finish_task_switch+0x1e3/0x240 kernel/sched/core.c:2493
 [<     inline     >] ? context_switch kernel/sched/core.c:2614
 [<ffffffff81edd361>] ? __schedule+0x401/0xb20 kernel/sched/core.c:3076
 [<ffffffff81261976>] ? SyS_write+0xa6/0xd0 include/linux/file.h:39
 [<ffffffff81eddab2>] ? schedule+0x32/0x80 kernel/sched/core.c:3106 (discriminator 1)
 [<ffffffff81ee3f83>] ? int_careful+0x14/0x22 arch/x86/entry/entry_64.S:303
RIP  [<     inline     >] kt_trace_add_event mm/ktsan/ktsan.h:512
RIP  [<ffffffff8125adba>] kt_thr_start+0xaa/0xb0 mm/ktsan/thr.c:124
 RSP <ffff88048078be40>
---[ end trace 08d41ce765fd8efc ]---

This is BUG_ON(thr->cpu->thr != NULL) in kt_thr_start.
How do we run 2 threads on the same cpu?..