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Subjectrcu: NULL ptr deref on boot
Hi Paul,

I've noticed the following on boot with the latest -next kernel:

[ 0.000000] WARNING: CPU: 0 PID: 0 at arch/x86/kernel/cpu/common.c:1439 warn_pre_alternatives+0x1e/0x20()
[ 0.000000] You're using static_cpu_has before alternatives have run!
[ 0.000000] Modules linked in:
[ 0.000000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.16.0-rc2-next-20140627-sasha-00023-g5d10814-dirty #734
[ 0.000000] 0000000000000009 ffffffff9d003c48 ffffffff9b525423 0000000000000002
[ 0.000000] ffffffff9d003c98 ffffffff9d003c88 ffffffff98168aec ffffffff9d003d58
[ 0.000000] 0000000000000000 ffffffff9d003e78 0000000000000000 0000000000000002
[ 0.000000] Call Trace:
[ 0.000000] dump_stack (lib/dump_stack.c:52)
[ 0.000000] warn_slowpath_common (kernel/panic.c:431)
[ 0.000000] warn_slowpath_fmt (kernel/panic.c:446)
[ 0.000000] ? irq_return (arch/x86/kernel/entry_64.S:842)
[ 0.000000] warn_pre_alternatives (arch/x86/kernel/cpu/common.c:1440)
[ 0.000000] __do_page_fault (./arch/x86/include/asm/cpufeature.h:423 arch/x86/mm/fault.c:1022 arch/x86/mm/fault.c:1112)
[ 0.000000] ? trace_hardirqs_off_caller (kernel/locking/lockdep.c:2638 (discriminator 2))
[ 0.000000] ? trace_hardirqs_off (kernel/locking/lockdep.c:2645)
[ 0.000000] ? __slab_alloc (mm/slub.c:2364 (discriminator 1))
[ 0.000000] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63)
[ 0.000000] ? trace_hardirqs_off_caller (kernel/locking/lockdep.c:2638 (discriminator 2))
[ 0.000000] ? error_sti (arch/x86/kernel/entry_64.S:1419)
[ 0.000000] trace_do_page_fault (arch/x86/mm/fault.c:1313 include/linux/jump_label.h:115 include/linux/context_tracking_state.h:27 include/linux/context_tracking.h:45 arch/x86/mm/fault.c:1314)
[ 0.000000] do_async_page_fault (arch/x86/kernel/kvm.c:264)
[ 0.000000] async_page_fault (arch/x86/kernel/entry_64.S:1322)
[ 0.000000] ? tick_nohz_init (include/linux/bitmap.h:165 include/linux/cpumask.h:333 kernel/time/tick-sched.c:344 kernel/time/tick-sched.c:356)
[ 0.000000] ? tick_nohz_init (include/linux/bitmap.h:164 include/linux/cpumask.h:333 kernel/time/tick-sched.c:344 kernel/time/tick-sched.c:356)
[ 0.000000] start_kernel (init/main.c:581)
[ 0.000000] ? set_init_arg (init/main.c:281)
[ 0.000000] ? early_idt_handlers (arch/x86/kernel/head_64.S:340)
[ 0.000000] x86_64_start_reservations (arch/x86/kernel/head64.c:194)
[ 0.000000] x86_64_start_kernel (arch/x86/kernel/head64.c:183)
[ 0.000000] ---[ end trace 4d5ff9f2f68c4233 ]---
[ 0.000000] BUG: unable to handle kernel NULL pointer dereference at (null)
[ 0.000000] IP: tick_nohz_init (include/linux/bitmap.h:165 include/linux/cpumask.h:333 kernel/time/tick-sched.c:344 kernel/time/tick-sched.c:356)
[ 0.000000] PGD 0
[ 0.000000] Oops: 0002 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 0.000000] Modules linked in:
[ 0.000000] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G W 3.16.0-rc2-next-20140627-sasha-00023-g5d10814-dirty #734
[ 0.000000] task: ffffffff9d0354c0 ti: ffffffff9d000000 task.ti: ffffffff9d000000
[ 0.000000] RIP: tick_nohz_init (include/linux/bitmap.h:165 include/linux/cpumask.h:333 kernel/time/tick-sched.c:344 kernel/time/tick-sched.c:356)
[ 0.000000] RSP: 0000:ffffffff9d003f28 EFLAGS: 00010002
[ 0.000000] RAX: 0000000000000000 RBX: ffff88003684d480 RCX: 0000000000000008
[ 0.000000] RDX: 0000000000000014 RSI: ffff88003684d480 RDI: 0000000000000000
[ 0.000000] RBP: ffffffff9d003f38 R08: ffff88003684d480 R09: ffff88003684d480
[ 0.000000] R10: ffff88003684d480 R11: 0000000000000001 R12: ffffffff9e5fd020
[ 0.000000] R13: ffff88070282ca00 R14: ffffffff9e607ae0 R15: 00000000000146f0
[ 0.000000] FS: 0000000000000000(0000) GS:ffff880036e00000(0000) knlGS:0000000000000000
[ 0.000000] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 0.000000] CR2: 0000000000000000 CR3: 000000001d02e000 CR4: 00000000000006b0
[ 0.000000] Stack:
[ 0.000000] ffffffffffffffff ffffffff9e5fd020 ffffffff9d003f88 ffffffff9e4c9f09
[ 0.000000] ffffffff9e4c98fd 00000000000146f0 ffffffff9d003f78 ffffffff9e607ae0
[ 0.000000] 0000000000000020 ffffffff9e4c9117 00000000ffffffff 0000ffffffff9e4c
[ 0.000000] Call Trace:
[ 0.000000] start_kernel (init/main.c:581)
[ 0.000000] ? set_init_arg (init/main.c:281)
[ 0.000000] ? early_idt_handlers (arch/x86/kernel/head_64.S:340)
[ 0.000000] x86_64_start_reservations (arch/x86/kernel/head64.c:194)
[ 0.000000] x86_64_start_kernel (arch/x86/kernel/head64.c:183)
[ 0.000000] Code: e8 d0 84 66 fa 89 c7 48 89 de e8 b0 46 02 fd 48 63 0d 4f 91 dd ff 31 c0 48 8b 3d 5e 1f 1c 01 48 83 c1 3f 48 c1 f9 03 48 83 e1 f8 <f3> aa 48 8b 1d 49 1f 1c 01 e8 9c 84 66 fa 89 c7 e8 25 3c d1 f9
All code
========
0: e8 d0 84 66 fa callq 0xfffffffffa6684d5
5: 89 c7 mov %eax,%edi
7: 48 89 de mov %rbx,%rsi
a: e8 b0 46 02 fd callq 0xfffffffffd0246bf
f: 48 63 0d 4f 91 dd ff movslq -0x226eb1(%rip),%rcx # 0xffffffffffdd9165
16: 31 c0 xor %eax,%eax
18: 48 8b 3d 5e 1f 1c 01 mov 0x11c1f5e(%rip),%rdi # 0x11c1f7d
1f: 48 83 c1 3f add $0x3f,%rcx
23: 48 c1 f9 03 sar $0x3,%rcx
27: 48 83 e1 f8 and $0xfffffffffffffff8,%rcx
2b: f3 aa rep stos %al,%es:*(%rdi) <-- trapping instruction
2d: 48 8b 1d 49 1f 1c 01 mov 0x11c1f49(%rip),%rbx # 0x11c1f7d
34: e8 9c 84 66 fa callq 0xfffffffffa6684d5
39: 89 c7 mov %eax,%edi
3b: e8 25 3c d1 f9 callq 0xfffffffff9d13c65
...

Code starting with the faulting instruction
===========================================
0: f3 aa rep stos %al,%es:(%rdi)
2: 48 8b 1d 49 1f 1c 01 mov 0x11c1f49(%rip),%rbx # 0x11c1f52
9: e8 9c 84 66 fa callq 0xfffffffffa6684aa
e: 89 c7 mov %eax,%edi
10: e8 25 3c d1 f9 callq 0xfffffffff9d13c3a
...
[ 0.000000] RIP tick_nohz_init (include/linux/bitmap.h:165 include/linux/cpumask.h:333 kernel/time/tick-sched.c:344 kernel/time/tick-sched.c:356)
[ 0.000000] RSP <ffffffff9d003f28>
[ 0.000000] CR2: 0000000000000000

Bisection pointed me to "rcu: Bind grace-period kthreads to non-NO_HZ_FULL CPUs".


Thanks,
Sasha


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