[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@xxxxxxxxx>
Signed-off-by: Yury Norov <ynorov@xxxxxxxxxx>
---
v1: https://lore.kernel.org/all/20260724232325.594212-1-ynorov@xxxxxxxxxx/
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@xxxxxxxxxx>
  */
+#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