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Subject[PATCH] arm64: use raw_smp_processor_id in stack backtrace dump
dump_backtrace may be called in kthread context, which is not bound to a single
cpu, i.e. khungtaskd, then calling smp_processor_id may trigger the below bug

BUG: using smp_processor_id() in preemptible [00000000] code: khungtaskd/71
caller is debug_smp_processor_id+0x1c/0x28
CPU: 3 PID: 71 Comm: khungtaskd Not tainted 4.5.0-rc1 #144
Hardware name: Freescale Layerscape 2085a RDB Board (DT)
Call trace:
[<ffffffc00008d120>] dump_backtrace+0x0/0x290
[<ffffffc00008d3d4>] show_stack+0x24/0x30
[<ffffffc0006bfd1c>] dump_stack+0x8c/0xd8
[<ffffffc0006fe56c>] check_preemption_disabled+0x184/0x188
[<ffffffc0006fe58c>] debug_smp_processor_id+0x1c/0x28
[<ffffffc00008d1a4>] dump_backtrace+0x84/0x290
[<ffffffc00008d3d4>] show_stack+0x24/0x30
[<ffffffc00012aa5c>] sched_show_task+0x16c/0x280
[<ffffffc0001f3df8>] watchdog+0x560/0x708
[<ffffffc000111bb8>] kthread+0x1b0/0x1d0
[<ffffffc0000864d0>] ret_from_fork+0x10/0x40

Replace it to raw version to prevent from the race condition.

Signed-off-by: Yang Shi <>
arch/arm64/kernel/traps.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index cbedd72..9abe236 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -146,7 +146,7 @@ static void dump_instr(const char *lvl, struct pt_regs *regs)
static void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
struct stackframe frame;
- unsigned long irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
+ unsigned long irq_stack_ptr = IRQ_STACK_PTR(raw_smp_processor_id());
int skip;

pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);
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