[PATCH] x86/smpboot: Mark native_play_dead() as __noreturn

[PATCH] x86/smpboot: Mark native_play_dead() as __noreturn

[Thorsten Blum]

native_play_dead() ends by calling the non-returning function
hlt_play_dead() and therefore also never returns.

The !CONFIG_HOTPLUG_CPU stub version of native_play_dead()
unconditionally calls BUG() and does not return either.

Add the __noreturn attribute to both function definitions and their
declaration to document this behavior and to potentially improve
compiler optimizations.

Remove the obsolete comment, and add native_play_dead() to the objtool's
list of __noreturn functions.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
---
 arch/x86/include/asm/smp.h | 2 +-
 arch/x86/kernel/smpboot.c  | 8 ++------
 tools/objtool/noreturns.h  | 1 +
 3 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index 22bfebe6776d..84951572ab81 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -109,7 +109,7 @@ int common_cpu_up(unsigned int cpunum, struct task_struct *tidle);
 int native_kick_ap(unsigned int cpu, struct task_struct *tidle);
 int native_cpu_disable(void);
 void __noreturn hlt_play_dead(void);
-void native_play_dead(void);
+void __noreturn native_play_dead(void);
 void play_dead_common(void);
 void wbinvd_on_cpu(int cpu);
 void wbinvd_on_all_cpus(void);
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index eb289abece23..a4ba735842a8 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1328,11 +1328,7 @@ void __noreturn hlt_play_dead(void)
 		native_halt();
 }
 
-/*
- * native_play_dead() is essentially a __noreturn function, but it can't
- * be marked as such as the compiler may complain about it.
- */
-void native_play_dead(void)
+void __noreturn native_play_dead(void)
 {
 	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS))
 		__update_spec_ctrl(0);
@@ -1351,7 +1347,7 @@ int native_cpu_disable(void)
 	return -ENOSYS;
 }
 
-void native_play_dead(void)
+void __noreturn native_play_dead(void)
 {
 	BUG();
 }
diff --git a/tools/objtool/noreturns.h b/tools/objtool/noreturns.h
index 802895fae3ca..14f8ab653449 100644
--- a/tools/objtool/noreturns.h
+++ b/tools/objtool/noreturns.h
@@ -36,6 +36,7 @@ NORETURN(machine_real_restart)
 NORETURN(make_task_dead)
 NORETURN(mpt_halt_firmware)
 NORETURN(mwait_play_dead)
+NORETURN(native_play_dead)
 NORETURN(nmi_panic_self_stop)
 NORETURN(panic)
 NORETURN(vpanic)
-- 
2.51.0

Re: [PATCH] x86/smpboot: Mark native_play_dead() as __noreturn

[Josh Poimboeuf]

On Mon, Oct 27, 2025 at 04:51:02PM +0100, Thorsten Blum wrote:
> native_play_dead() ends by calling the non-returning function
> hlt_play_dead() and therefore also never returns.
> 
> The !CONFIG_HOTPLUG_CPU stub version of native_play_dead()
> unconditionally calls BUG() and does not return either.
> 
> Add the __noreturn attribute to both function definitions and their
> declaration to document this behavior and to potentially improve
> compiler optimizations.
> 
> Remove the obsolete comment, and add native_play_dead() to the objtool's
> list of __noreturn functions.
> 
> Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>

Is there an objtool warning that this fixes?  If so, it would be helpful
to put it in the description above.

And same for the other patch.

Otherwise it LGTM.  Thanks!

-- 
Josh

Re: [PATCH] x86/smpboot: Mark native_play_dead() as __noreturn

[Thorsten Blum]

On 27. Oct 2025, at 20:28, Josh Poimboeuf wrote:
> On Mon, Oct 27, 2025 at 04:51:02PM +0100, Thorsten Blum wrote:
>> native_play_dead() ends by calling the non-returning function
>> hlt_play_dead() and therefore also never returns.
>> 
>> The !CONFIG_HOTPLUG_CPU stub version of native_play_dead()
>> unconditionally calls BUG() and does not return either.
>> 
>> Add the __noreturn attribute to both function definitions and their
>> declaration to document this behavior and to potentially improve
>> compiler optimizations.
>> 
>> Remove the obsolete comment, and add native_play_dead() to the objtool's
>> list of __noreturn functions.
>> 
>> Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
> 
> Is there an objtool warning that this fixes?  If so, it would be helpful
> to put it in the description above.

Not that I'm aware of.

Re: [PATCH] x86/smpboot: Mark native_play_dead() as __noreturn

[Josh Poimboeuf]

On Mon, Oct 27, 2025 at 08:56:18PM +0100, Thorsten Blum wrote:
> On 27. Oct 2025, at 20:28, Josh Poimboeuf wrote:
> > On Mon, Oct 27, 2025 at 04:51:02PM +0100, Thorsten Blum wrote:
> >> native_play_dead() ends by calling the non-returning function
> >> hlt_play_dead() and therefore also never returns.
> >> 
> >> The !CONFIG_HOTPLUG_CPU stub version of native_play_dead()
> >> unconditionally calls BUG() and does not return either.
> >> 
> >> Add the __noreturn attribute to both function definitions and their
> >> declaration to document this behavior and to potentially improve
> >> compiler optimizations.
> >> 
> >> Remove the obsolete comment, and add native_play_dead() to the objtool's
> >> list of __noreturn functions.
> >> 
> >> Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
> > 
> > Is there an objtool warning that this fixes?  If so, it would be helpful
> > to put it in the description above.
> 
> Not that I'm aware of.

Ok.

Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>

-- 
Josh

[tip: objtool/core] x86/smpboot: Mark native_play_dead() as __noreturn

[tip-bot2 for Thorsten Blum]

The following commit has been merged into the objtool/core branch of tip:

Commit-ID:     0ccf30fc64acca8e43a54a4f54fb3a4f155d4692
Gitweb:        https://git.kernel.org/tip/0ccf30fc64acca8e43a54a4f54fb3a4f155d4692
Author:        Thorsten Blum <thorsten.blum@linux.dev>
AuthorDate:    Mon, 27 Oct 2025 16:51:02 +01:00
Committer:     Josh Poimboeuf <jpoimboe@kernel.org>
CommitterDate: Thu, 30 Oct 2025 08:29:41 -07:00

x86/smpboot: Mark native_play_dead() as __noreturn

native_play_dead() ends by calling the non-returning function
hlt_play_dead() and therefore also never returns.

The !CONFIG_HOTPLUG_CPU stub version of native_play_dead()
unconditionally calls BUG() and does not return either.

Add the __noreturn attribute to both function definitions and their
declaration to document this behavior and to potentially improve
compiler optimizations.

Remove the obsolete comment, and add native_play_dead() to the objtool's
list of __noreturn functions.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20251027155107.183136-1-thorsten.blum@linux.dev
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/x86/include/asm/smp.h | 2 +-
 arch/x86/kernel/smpboot.c  | 8 ++------
 tools/objtool/noreturns.h  | 1 +
 3 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index 22bfebe..8495157 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -109,7 +109,7 @@ int common_cpu_up(unsigned int cpunum, struct task_struct *tidle);
 int native_kick_ap(unsigned int cpu, struct task_struct *tidle);
 int native_cpu_disable(void);
 void __noreturn hlt_play_dead(void);
-void native_play_dead(void);
+void __noreturn native_play_dead(void);
 void play_dead_common(void);
 void wbinvd_on_cpu(int cpu);
 void wbinvd_on_all_cpus(void);
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index eb289ab..a4ba735 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1328,11 +1328,7 @@ void __noreturn hlt_play_dead(void)
 		native_halt();
 }
 
-/*
- * native_play_dead() is essentially a __noreturn function, but it can't
- * be marked as such as the compiler may complain about it.
- */
-void native_play_dead(void)
+void __noreturn native_play_dead(void)
 {
 	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS))
 		__update_spec_ctrl(0);
@@ -1351,7 +1347,7 @@ int native_cpu_disable(void)
 	return -ENOSYS;
 }
 
-void native_play_dead(void)
+void __noreturn native_play_dead(void)
 {
 	BUG();
 }
diff --git a/tools/objtool/noreturns.h b/tools/objtool/noreturns.h
index 802895f..14f8ab6 100644
--- a/tools/objtool/noreturns.h
+++ b/tools/objtool/noreturns.h
@@ -36,6 +36,7 @@ NORETURN(machine_real_restart)
 NORETURN(make_task_dead)
 NORETURN(mpt_halt_firmware)
 NORETURN(mwait_play_dead)
+NORETURN(native_play_dead)
 NORETURN(nmi_panic_self_stop)
 NORETURN(panic)
 NORETURN(vpanic)