[PATCH 1/2] kthread: Move prio/affinite change into the newly created thread

[Sebastian Andrzej Siewior]

With enabled threaded interrupts the nouveau driver reported the
following:
| Chain exists of:
|   &mm->mmap_lock#2 --> &device->mutex --> &cpuset_rwsem
|
|  Possible unsafe locking scenario:
|
|        CPU0                    CPU1
|        ----                    ----
|   lock(&cpuset_rwsem);
|                                lock(&device->mutex);
|                                lock(&cpuset_rwsem);
|   lock(&mm->mmap_lock#2);

The device->mutex is nvkm_device::mutex.

Unblocking the lockchain at `cpuset_rwsem' is probably the easiest thing
to do.
Move the priority reset to the start of the newly created thread.

Fixes: 710da3c8ea7df ("sched/core: Prevent race condition between cpuset and 
__sched_setscheduler()")
Reported-by: Mike Galbraith <efa...@gmx.de>
Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
Link: 
https://lkml.kernel.org/r/a23a826af7c108ea5651e73b8fbae5e653f16e86.ca...@gmx.de
---
 kernel/kthread.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 933a625621b8d..4a31127c6efbf 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -243,6 +243,7 @@ EXPORT_SYMBOL_GPL(kthread_parkme);
 
 static int kthread(void *_create)
 {
+       static const struct sched_param param = { .sched_priority = 0 };
        /* Copy data: it's on kthread's stack */
        struct kthread_create_info *create = _create;
        int (*threadfn)(void *data) = create->threadfn;
@@ -273,6 +274,13 @@ static int kthread(void *_create)
        init_completion(&self->parked);
        current->vfork_done = &self->exited;
 
+       /*
+        * The new thread inherited kthreadd's priority and CPU mask. Reset
+        * back to default in case they have been changed.
+        */
+       sched_setscheduler_nocheck(current, SCHED_NORMAL, &param);
+       set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_KTHREAD));
+
        /* OK, tell user we're spawned, wait for stop or wakeup */
        __set_current_state(TASK_UNINTERRUPTIBLE);
        create->result = current;
@@ -370,7 +378,6 @@ struct task_struct *__kthread_create_on_node(int 
(*threadfn)(void *data),
        }
        task = create->result;
        if (!IS_ERR(task)) {
-               static const struct sched_param param = { .sched_priority = 0 };
                char name[TASK_COMM_LEN];
 
                /*
@@ -379,13 +386,6 @@ struct task_struct *__kthread_create_on_node(int 
(*threadfn)(void *data),
                 */
                vsnprintf(name, sizeof(name), namefmt, args);
                set_task_comm(task, name);
-               /*
-                * root may have changed our (kthreadd's) priority or CPU mask.
-                * The kernel thread should not inherit these properties.
-                */
-               sched_setscheduler_nocheck(task, SCHED_NORMAL, &param);
-               set_cpus_allowed_ptr(task,
-                                    housekeeping_cpumask(HK_FLAG_KTHREAD));
        }
        kfree(create);
        return task;
-- 
2.29.2