[PATCH v15 02/12] cpu/hotplug: Move idle_thread_get() to <linux/smpboot.h>

[Usama Arif <usama.arif@xxxxxxxxxxxxx>]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Instead of relying purely on the special-case wrapper in bringup_cpu()
to pass the idle thread to __cpu_up(), expose idle_thread_get() so that
the architecture code can obtain it directly when necessary.

This will be useful when the existing __cpu_up() is split into multiple
phases, only *one* of which will actually need the idle thread.

If the architecture code is to register its new pre-bringup states with
the cpuhp core, having a special-case wrapper to pass extra arguments is
non-trivial and it's easier just to let the arch register its function
pointer to be invoked with the standard API.

To reduce duplication, move the shadow stack reset and kasan unpoisoning
into idle_thread_get() too.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Usama Arif <usama.arif@xxxxxxxxxxxxx>
Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Tested-by: Kim Phillips <kim.phillips@xxxxxxx>
Tested-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx>
Tested-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx>
---
 include/linux/smpboot.h | 10 ++++++++++
 kernel/cpu.c            | 13 +++----------
 kernel/smpboot.c        | 11 ++++++++++-
 kernel/smpboot.h        |  2 --
 4 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/include/linux/smpboot.h b/include/linux/smpboot.h
index 9d1bc65d226c..df6417703e4c 100644
--- a/include/linux/smpboot.h
+++ b/include/linux/smpboot.h
@@ -5,6 +5,16 @@
 #include <linux/types.h>
 
 struct task_struct;
+
+#ifdef CONFIG_GENERIC_SMP_IDLE_THREAD
+struct task_struct *idle_thread_get(unsigned int cpu, bool unpoison);
+#else
+static inline struct task_struct *idle_thread_get(unsigned int cpu, bool unpoison)
+{
+	return NULL;
+}
+#endif
+
 /* Cookie handed to the thread_fn*/
 struct smpboot_thread_data;
 
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6c0a92ca6bb5..6b3dccb4a888 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -31,7 +31,6 @@
 #include <linux/smpboot.h>
 #include <linux/relay.h>
 #include <linux/slab.h>
-#include <linux/scs.h>
 #include <linux/percpu-rwsem.h>
 #include <linux/cpuset.h>
 #include <linux/random.h>
@@ -588,15 +587,9 @@ static int bringup_wait_for_ap(unsigned int cpu)
 
 static int bringup_cpu(unsigned int cpu)
 {
-	struct task_struct *idle = idle_thread_get(cpu);
+	struct task_struct *idle = idle_thread_get(cpu, true);
 	int ret;
 
-	/*
-	 * Reset stale stack state from the last time this CPU was online.
-	 */
-	scs_task_reset(idle);
-	kasan_unpoison_task_stack(idle);
-
 	/*
 	 * Some architectures have to walk the irq descriptors to
 	 * setup the vector space for the cpu which comes online.
@@ -614,7 +607,7 @@ static int bringup_cpu(unsigned int cpu)
 
 static int finish_cpu(unsigned int cpu)
 {
-	struct task_struct *idle = idle_thread_get(cpu);
+	struct task_struct *idle = idle_thread_get(cpu, false);
 	struct mm_struct *mm = idle->active_mm;
 
 	/*
@@ -1378,7 +1371,7 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
 
 	if (st->state == CPUHP_OFFLINE) {
 		/* Let it fail before we try to bring the cpu up */
-		idle = idle_thread_get(cpu);
+		idle = idle_thread_get(cpu, false);
 		if (IS_ERR(idle)) {
 			ret = PTR_ERR(idle);
 			goto out;
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 2c7396da470c..24e81c725e7b 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -11,6 +11,7 @@
 #include <linux/slab.h>
 #include <linux/sched.h>
 #include <linux/sched/task.h>
+#include <linux/scs.h>
 #include <linux/export.h>
 #include <linux/percpu.h>
 #include <linux/kthread.h>
@@ -27,12 +28,20 @@
  */
 static DEFINE_PER_CPU(struct task_struct *, idle_threads);
 
-struct task_struct *idle_thread_get(unsigned int cpu)
+struct task_struct *idle_thread_get(unsigned int cpu, bool unpoison)
 {
 	struct task_struct *tsk = per_cpu(idle_threads, cpu);
 
 	if (!tsk)
 		return ERR_PTR(-ENOMEM);
+
+	if (unpoison) {
+		/*
+		 * Reset stale stack state from last time this CPU was online.
+		 */
+		scs_task_reset(tsk);
+		kasan_unpoison_task_stack(tsk);
+	}
 	return tsk;
 }
 
diff --git a/kernel/smpboot.h b/kernel/smpboot.h
index 34dd3d7ba40b..60c609318ad6 100644
--- a/kernel/smpboot.h
+++ b/kernel/smpboot.h
@@ -5,11 +5,9 @@
 struct task_struct;
 
 #ifdef CONFIG_GENERIC_SMP_IDLE_THREAD
-struct task_struct *idle_thread_get(unsigned int cpu);
 void idle_thread_set_boot_cpu(void);
 void idle_threads_init(void);
 #else
-static inline struct task_struct *idle_thread_get(unsigned int cpu) { return NULL; }
 static inline void idle_thread_set_boot_cpu(void) { }
 static inline void idle_threads_init(void) { }
 #endif
-- 
2.25.1