[PATCH v17 2/8] cpu/hotplug: Reset task stack state in _cpu_up()

[Usama Arif]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Commit dce1ca0525bf ("sched/scs: Reset task stack state in bringup_cpu()")
ensured that the shadow call stack was reset and KASAN poisoning removed
from a CPU's stack each time that CPU is brought up, not just once.

This is not incorrect. However, with parallel bringup, an architecture
may obtain the idle thread for a new CPU from a pre-bringup stage, by
calling idle_thread_get() for itself. This would mean that the cleanup
in bringup_cpu() would be too late.

Move the SCS/KASAN cleanup to the generic _cpu_up() function instead,
which already ensures that the new CPU's stack is available, purely to
allow for early failure. This occurs when the CPU to be brought up is
in the CPUHP_OFFLINE state, which should correctly do the cleanup any
time the CPU has been taken down to the point where such is needed.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
Tested-by: Mark Rutland <mark.rutland@xxxxxxx> [arm64]
---
 kernel/cpu.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6c0a92ca6bb5..43e0a77f21e8 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -591,12 +591,6 @@ static int bringup_cpu(unsigned int cpu)
 	struct task_struct *idle = idle_thread_get(cpu);
 	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.
@@ -1383,6 +1377,12 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
 			ret = PTR_ERR(idle);
 			goto out;
 		}
+
+		/*
+		 * Reset stale stack state from the last time this CPU was online.
+		 */
+		scs_task_reset(idle);
+		kasan_unpoison_task_stack(idle);
 	}
 
 	cpuhp_tasks_frozen = tasks_frozen;
-- 
2.25.1