[PATCH v2] stop_machine: Make stop_one_cpu_nowait() return void

[PATCH v2] stop_machine: Make stop_one_cpu_nowait() return void

[Yury Norov]

No caller checks the return value from stop_one_cpu_nowait(). All
callers require the callback to run and arrange for the target CPU's
stopper to remain enabled while queuing the work. In particular, commit
f0498d2a54e7 ("sched: Fix stop_one_cpu_nowait() vs hotplug") added
preemption protection to the scheduler callers so that queuing must
succeed once the target CPU has been observed online.

Therefore, a failure is an unrecoverable violation rather than a condition
individual callers can recover from. Diagnose it with WARN_ON_ONCE() in
stop_one_cpu_nowait(). A check in the common helper covers current and
future callers consistently, while individual checks would duplicate
the same non-recoverable handling at every call site.

Make the function return void because there is no longer a meaningful
result for callers to consume.

On UP, warn if the supplied CPU is not the current CPU because the work
cannot be scheduled in that case.

CC: Bradley Morgan <include@grrlz.net>
Signed-off-by: Yury Norov <ynorov@nvidia.com>
---
v1: https://lore.kernel.org/all/20260724232325.594212-1-ynorov@nvidia.com/
v2:
 - carefully include linux/bug.h (Bradley);
 - update the CONTEXT section (Bradley).

 include/linux/stop_machine.h | 21 ++++++++++-----------
 kernel/stop_machine.c        | 14 ++++++--------
 2 files changed, 16 insertions(+), 19 deletions(-)

diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h
index 01011113d226..84e7fb627ba4 100644
--- a/include/linux/stop_machine.h
+++ b/include/linux/stop_machine.h
@@ -2,6 +2,7 @@
 #ifndef _LINUX_STOP_MACHINE
 #define _LINUX_STOP_MACHINE
 
+#include <linux/bug.h>
 #include <linux/cpu.h>
 #include <linux/cpumask_types.h>
 #include <linux/smp.h>
@@ -31,7 +32,7 @@ struct cpu_stop_work {
 
 int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg);
 int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg);
-bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
+void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
 			 struct cpu_stop_work *work_buf);
 void stop_machine_park(int cpu);
 void stop_machine_unpark(int cpu);
@@ -68,19 +69,17 @@ static void stop_one_cpu_nowait_workfn(struct work_struct *work)
 	preempt_enable();
 }
 
-static inline bool stop_one_cpu_nowait(unsigned int cpu,
+static inline void stop_one_cpu_nowait(unsigned int cpu,
 				       cpu_stop_fn_t fn, void *arg,
 				       struct cpu_stop_work *work_buf)
 {
-	if (cpu == smp_processor_id()) {
-		INIT_WORK(&work_buf->work, stop_one_cpu_nowait_workfn);
-		work_buf->fn = fn;
-		work_buf->arg = arg;
-		schedule_work(&work_buf->work);
-		return true;
-	}
-
-	return false;
+	if (WARN_ON_ONCE(cpu != smp_processor_id()))
+		return;
+
+	INIT_WORK(&work_buf->work, stop_one_cpu_nowait_workfn);
+	work_buf->fn = fn;
+	work_buf->arg = arg;
+	schedule_work(&work_buf->work);
 }
 
 static inline void print_stop_info(const char *log_lvl, struct task_struct *task) { }
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 773d8e9ae30c..d085ba1f4b44 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -7,6 +7,7 @@
  * Copyright (C) 2010		SUSE Linux Products GmbH
  * Copyright (C) 2010		Tejun Heo <tj@kernel.org>
  */
+#include <linux/bug.h>
 #include <linux/compiler.h>
 #include <linux/completion.h>
 #include <linux/cpu.h>
@@ -376,17 +377,14 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *
  * and will remain untouched until stopper starts executing @fn.
  *
  * CONTEXT:
- * Don't care.
- *
- * RETURNS:
- * true if cpu_stop_work was queued successfully and @fn will be called,
- * false otherwise.
+ * Don't care, but the caller must ensure @cpu's stopper stays enabled
+ * until the work is queued, e.g. by preempt_disable().
  */
-bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
-			struct cpu_stop_work *work_buf)
+void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
+			 struct cpu_stop_work *work_buf)
 {
 	*work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, .caller = _RET_IP_, };
-	return cpu_stop_queue_work(cpu, work_buf);
+	WARN_ON_ONCE(!cpu_stop_queue_work(cpu, work_buf));
 }
 
 static bool queue_stop_cpus_work(const struct cpumask *cpumask,
-- 
2.53.0

Re: [PATCH v2] stop_machine: Make stop_one_cpu_nowait() return void

[Bradley Morgan]

On 29 July 2026 03:23:54 BST, Yury Norov <ynorov@nvidia.com> wrote:
>No caller checks the return value from stop_one_cpu_nowait(). All
>callers require the callback to run and arrange for the target CPU's
>stopper to remain enabled while queuing the work. In particular, commit
>f0498d2a54e7 ("sched: Fix stop_one_cpu_nowait() vs hotplug") added
>preemption protection to the scheduler callers so that queuing must
>succeed once the target CPU has been observed online.
>
>Therefore, a failure is an unrecoverable violation rather than a condition
>individual callers can recover from. Diagnose it with WARN_ON_ONCE() in
>stop_one_cpu_nowait(). A check in the common helper covers current and
>future callers consistently, while individual checks would duplicate
>the same non-recoverable handling at every call site.
>
>Make the function return void because there is no longer a meaningful
>result for callers to consume.
>
>On UP, warn if the supplied CPU is not the current CPU because the work
>cannot be scheduled in that case.
>
>CC: Bradley Morgan <include@grrlz.net>
Reviewed-by: Bradley Morgan <include@grrlz.net>
>Signed-off-by: Yury Norov <ynorov@nvidia.com>
>---
>v1: https://lore.kernel.org/all/20260724232325.594212-1-ynorov@nvidia.com/
>v2:
> - carefully include linux/bug.h (Bradley);
> - update the CONTEXT section (Bradley).
>
> include/linux/stop_machine.h | 21 ++++++++++-----------
> kernel/stop_machine.c        | 14 ++++++--------
> 2 files changed, 16 insertions(+), 19 deletions(-)
>
>diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h
>index 01011113d226..84e7fb627ba4 100644
>--- a/include/linux/stop_machine.h
>+++ b/include/linux/stop_machine.h
>@@ -2,6 +2,7 @@
> #ifndef _LINUX_STOP_MACHINE
> #define _LINUX_STOP_MACHINE
> 
>+#include <linux/bug.h>
> #include <linux/cpu.h>
> #include <linux/cpumask_types.h>
> #include <linux/smp.h>
>@@ -31,7 +32,7 @@ struct cpu_stop_work {
> 
> int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg);
> int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn,
> void *arg);
>-bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
>+void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
> 			 struct cpu_stop_work *work_buf);
> void stop_machine_park(int cpu);
> void stop_machine_unpark(int cpu);
>@@ -68,19 +69,17 @@ static void stop_one_cpu_nowait_workfn(struct work_struct *work)
> 	preempt_enable();
> }
> 
>-static inline bool stop_one_cpu_nowait(unsigned int cpu,
>+static inline void stop_one_cpu_nowait(unsigned int cpu,
> 				       cpu_stop_fn_t fn, void *arg,
> 				       struct cpu_stop_work *work_buf)
> {
>-	if (cpu == smp_processor_id()) {
>-		INIT_WORK(&work_buf->work, stop_one_cpu_nowait_workfn);
>-		work_buf->fn = fn;
>-		work_buf->arg = arg;
>-		schedule_work(&work_buf->work);
>-		return true;
>-	}
>-
>-	return false;
>+	if (WARN_ON_ONCE(cpu != smp_processor_id()))
>+		return;
>+
>+	INIT_WORK(&work_buf->work, stop_one_cpu_nowait_workfn);
>+	work_buf->fn = fn;
>+	work_buf->arg = arg;
>+	schedule_work(&work_buf->work);
> }
> 
> static inline void print_stop_info(const char *log_lvl, struct
> task_struct *task) { }
>diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
>index 773d8e9ae30c..d085ba1f4b44 100644
>--- a/kernel/stop_machine.c
>+++ b/kernel/stop_machine.c
>@@ -7,6 +7,7 @@
>  * Copyright (C) 2010		SUSE Linux Products GmbH
>  * Copyright (C) 2010		Tejun Heo <tj@kernel.org>
>  */
>+#include <linux/bug.h>
> #include <linux/compiler.h>
> #include <linux/completion.h>
> #include <linux/cpu.h>
>@@ -376,17 +377,14 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *
>  * and will remain untouched until stopper starts executing @fn.
>  *
>  * CONTEXT:
>- * Don't care.
>- *
>- * RETURNS:
>- * true if cpu_stop_work was queued successfully and @fn will be called,
>- * false otherwise.
>+ * Don't care, but the caller must ensure @cpu's stopper stays enabled
>+ * until the work is queued, e.g. by preempt_disable().
>  */
>-bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
>-			struct cpu_stop_work *work_buf)
>+void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
>+			 struct cpu_stop_work *work_buf)
> {
> 	*work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, .caller = _RET_IP_, };
>-	return cpu_stop_queue_work(cpu, work_buf);
>+	WARN_ON_ONCE(!cpu_stop_queue_work(cpu, work_buf));
> }
> 
> static bool queue_stop_cpus_work(const struct cpumask *cpumask,
>


Thanks!

Re: [PATCH v2] stop_machine: Make stop_one_cpu_nowait() return void

[Peter Zijlstra]

On Tue, Jul 28, 2026 at 10:23:54PM -0400, Yury Norov wrote:
> No caller checks the return value from stop_one_cpu_nowait(). All
> callers require the callback to run and arrange for the target CPU's
> stopper to remain enabled while queuing the work. In particular, commit
> f0498d2a54e7 ("sched: Fix stop_one_cpu_nowait() vs hotplug") added
> preemption protection to the scheduler callers so that queuing must
> succeed once the target CPU has been observed online.
> 
> Therefore, a failure is an unrecoverable violation rather than a condition
> individual callers can recover from. Diagnose it with WARN_ON_ONCE() in
> stop_one_cpu_nowait(). A check in the common helper covers current and
> future callers consistently, while individual checks would duplicate
> the same non-recoverable handling at every call site.
> 
> Make the function return void because there is no longer a meaningful
> result for callers to consume.
> 
> On UP, warn if the supplied CPU is not the current CPU because the work
> cannot be scheduled in that case.
> 
> CC: Bradley Morgan <include@grrlz.net>
> Signed-off-by: Yury Norov <ynorov@nvidia.com>

I suppose; you want me to take this through the scheduler tree?

Re: [PATCH v2] stop_machine: Make stop_one_cpu_nowait() return void

[Yury Norov]

On Wed, Jul 29, 2026 at 02:51:33PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 28, 2026 at 10:23:54PM -0400, Yury Norov wrote:
> > No caller checks the return value from stop_one_cpu_nowait(). All
> > callers require the callback to run and arrange for the target CPU's
> > stopper to remain enabled while queuing the work. In particular, commit
> > f0498d2a54e7 ("sched: Fix stop_one_cpu_nowait() vs hotplug") added
> > preemption protection to the scheduler callers so that queuing must
> > succeed once the target CPU has been observed online.
> > 
> > Therefore, a failure is an unrecoverable violation rather than a condition
> > individual callers can recover from. Diagnose it with WARN_ON_ONCE() in
> > stop_one_cpu_nowait(). A check in the common helper covers current and
> > future callers consistently, while individual checks would duplicate
> > the same non-recoverable handling at every call site.
> > 
> > Make the function return void because there is no longer a meaningful
> > result for callers to consume.
> > 
> > On UP, warn if the supplied CPU is not the current CPU because the work
> > cannot be scheduled in that case.
> > 
> > CC: Bradley Morgan <include@grrlz.net>
> > Signed-off-by: Yury Norov <ynorov@nvidia.com>
> 
> I suppose; you want me to take this through the scheduler tree?

Sure, thanks!

Re: [PATCH v2] stop_machine: Make stop_one_cpu_nowait() return void

[Shrikanth Hegde]

Hi Yury.

On 7/29/26 7:53 AM, Yury Norov wrote:
> No caller checks the return value from stop_one_cpu_nowait(). All
> callers require the callback to run and arrange for the target CPU's
> stopper to remain enabled while queuing the work. In particular, commit
> f0498d2a54e7 ("sched: Fix stop_one_cpu_nowait() vs hotplug") added
> preemption protection to the scheduler callers so that queuing must
> succeed once the target CPU has been observed online.
> 
> Therefore, a failure is an unrecoverable violation rather than a condition
> individual callers can recover from. Diagnose it with WARN_ON_ONCE() in
> stop_one_cpu_nowait(). A check in the common helper covers current and
> future callers consistently, while individual checks would duplicate
> the same non-recoverable handling at every call site.
> 
> Make the function return void because there is no longer a meaningful
> result for callers to consume.
> 
> On UP, warn if the supplied CPU is not the current CPU because the work
> cannot be scheduled in that case.
> 
> CC: Bradley Morgan <include@grrlz.net>
> Signed-off-by: Yury Norov <ynorov@nvidia.com>
> ---
> v1: https://lore.kernel.org/all/20260724232325.594212-1-ynorov@nvidia.com/
> v2:
>   - carefully include linux/bug.h (Bradley);
>   - update the CONTEXT section (Bradley).
> 
>   include/linux/stop_machine.h | 21 ++++++++++-----------
>   kernel/stop_machine.c        | 14 ++++++--------
>   2 files changed, 16 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h
> index 01011113d226..84e7fb627ba4 100644
> --- a/include/linux/stop_machine.h
> +++ b/include/linux/stop_machine.h
> @@ -2,6 +2,7 @@
>   #ifndef _LINUX_STOP_MACHINE
>   #define _LINUX_STOP_MACHINE
>   
> +#include <linux/bug.h>
>   #include <linux/cpu.h>
>   #include <linux/cpumask_types.h>
>   #include <linux/smp.h>
> @@ -31,7 +32,7 @@ struct cpu_stop_work {
>   
>   int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg);
>   int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg);
> -bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
> +void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
>   			 struct cpu_stop_work *work_buf);
>   void stop_machine_park(int cpu);
>   void stop_machine_unpark(int cpu);
> @@ -68,19 +69,17 @@ static void stop_one_cpu_nowait_workfn(struct work_struct *work)
>   	preempt_enable();
>   }
>   
> -static inline bool stop_one_cpu_nowait(unsigned int cpu,
> +static inline void stop_one_cpu_nowait(unsigned int cpu,
>   				       cpu_stop_fn_t fn, void *arg,
>   				       struct cpu_stop_work *work_buf)
>   {
> -	if (cpu == smp_processor_id()) {
> -		INIT_WORK(&work_buf->work, stop_one_cpu_nowait_workfn);
> -		work_buf->fn = fn;
> -		work_buf->arg = arg;
> -		schedule_work(&work_buf->work);
> -		return true;
> -	}
> -
> -	return false;
> +	if (WARN_ON_ONCE(cpu != smp_processor_id()))
> +		return;
> +
> +	INIT_WORK(&work_buf->work, stop_one_cpu_nowait_workfn);
> +	work_buf->fn = fn;
> +	work_buf->arg = arg;
> +	schedule_work(&work_buf->work);
>   }
>   
>   static inline void print_stop_info(const char *log_lvl, struct task_struct *task) { }
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 773d8e9ae30c..d085ba1f4b44 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -7,6 +7,7 @@
>    * Copyright (C) 2010		SUSE Linux Products GmbH
>    * Copyright (C) 2010		Tejun Heo <tj@kernel.org>
>    */
> +#include <linux/bug.h>
>   #include <linux/compiler.h>
>   #include <linux/completion.h>
>   #include <linux/cpu.h>
> @@ -376,17 +377,14 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *
>    * and will remain untouched until stopper starts executing @fn.
>    *
>    * CONTEXT:
> - * Don't care.
> - *
> - * RETURNS:
> - * true if cpu_stop_work was queued successfully and @fn will be called,
> - * false otherwise.
> + * Don't care, but the caller must ensure @cpu's stopper stays enabled
> + * until the work is queued, e.g. by preempt_disable().

nit:
* until the work is queued, i.e. ensuring preemption/irq disabled. ?

Because, if irq are disabled it is equally good enough.
#define preemptible()   (preempt_count() == 0 && !irqs_disabled())


>    */
> -bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
> -			struct cpu_stop_work *work_buf)
> +void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
> +			 struct cpu_stop_work *work_buf)
>   {
>   	*work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, .caller = _RET_IP_, };
> -	return cpu_stop_queue_work(cpu, work_buf);
> +	WARN_ON_ONCE(!cpu_stop_queue_work(cpu, work_buf));
>   }
>   
>   static bool queue_stop_cpus_work(const struct cpumask *cpumask,

Other than that,
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>