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Subject[PATCH 1/2] kthread: Move prio/affinite change into the newly created thread
Date
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 <efault@gmx.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/a23a826af7c108ea5651e73b8fbae5e653f16e86.camel@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
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