linux-kernel - Re: [PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings about entry functions

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Message-ID: <3da2092b-e6ca-a7a1-9459-c2754adf19aa@loongson.cn>
Date: Thu, 4 Sep 2025 11:18:28 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Huacai Chen <chenhuacai@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Nathan Chancellor <nathan@...nel.org>,
 loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings
 about entry functions

On 2025/9/4 上午3:22, Josh Poimboeuf wrote:
> On Mon, Sep 01, 2025 at 03:21:56PM +0800, Tiezhu Yang wrote:
>> +++ b/arch/loongarch/kernel/Makefile
>> @@ -3,8 +3,6 @@
>>   # Makefile for the Linux/LoongArch kernel.
>>   #
>>   
>> -OBJECT_FILES_NON_STANDARD_head.o := y
>> -
>>   always-$(KBUILD_BUILTIN)	:= vmlinux.lds
>>   
>>   obj-y		+= head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
>> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
>> index e3865e92a917..a11880f3a7e1 100644
>> --- a/arch/loongarch/kernel/head.S
>> +++ b/arch/loongarch/kernel/head.S
>> @@ -42,6 +42,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
>>   	.align 12
>>   
>>   SYM_CODE_START(kernel_entry)			# kernel entry point
>> +	UNWIND_HINT_UNDEFINED
> 
> Should this not be UNWIND_HINT_END_OF_STACK?

Yes, makes sense, will do it in the next version.

> I notice Loongarch doesn't seem to use that anywhere.  How does any ORC
> unwind succeed?  UNWIND_HINT_UNDEFINED sets an error condition which
> should cause a livepatch transition to stall.

Actually, kernel_entry() or smpboot_entry() is recognized as the last
frame, because at this point is_entry_func() is true and 
state->stack_info.type = STACK_TYPE_UNKNOWN in unwind_next_frame() of
arch/loongarch/kernel/unwind_orc.c.

Call Trace:
[<90000000031a36b4>] show_stack+0x5c/0x180
[<900000000319d4b0>] dump_stack_lvl+0x6c/0x9c
[<900000000458d3e0>] nmi_cpu_backtrace+0x160/0x168
[<90000000031a3b6c>] handle_backtrace+0xc/0x40
[<90000000032b1078>] __flush_smp_call_function_queue+0xd0/0x330
[<90000000031aec40>] loongson_ipi_interrupt+0xb0/0x168
[<900000000325b4d4>] __handle_irq_event_percpu+0x54/0x1a8
[<900000000325b63c>] handle_irq_event_percpu+0x14/0x80
[<9000000003262a48>] handle_percpu_irq+0x50/0xa0
[<900000000325aad4>] generic_handle_domain_irq+0x2c/0x80
[<9000000003bc4d24>] handle_cpu_irq+0x64/0xa0
[<90000000045a350c>] handle_loongarch_irq+0x2c/0x48
[<90000000045a35e4>] do_vint+0xbc/0xe0
[<90000000031a1624>] handle_vint+0x144/0x1e4
[<900000000321e2e8>] _nohz_idle_balance.isra.0+0x230/0x3a0
[<90000000031cd004>] handle_softirqs+0x10c/0x298
[<90000000031cd2e8>] __irq_exit_rcu+0x100/0x160
[<90000000045a35a4>] do_vint+0x7c/0xe0
[<90000000045a55cc>] idle_exit+0x0/0x4
[<90000000045a55d8>] arch_cpu_idle+0x8/0x30
[<90000000045a5698>] default_idle_call+0x18/0x50
[<90000000032250b0>] do_idle+0xb8/0x130
[<9000000003225374>] cpu_startup_entry+0x2c/0x38
[<90000000045a6240>] kernel_entry_end+0xdc/0xe0
[<90000000045c0d44>] start_kernel+0x65c/0x660
[<90000000045a60f0>] kernel_entry+0xf0/0xf8

Thanks,
Tiezhu