Commit 1a7243ca authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Peter Zijlstra
Browse files

kthread: Move prio/affinite change into the newly created thread 



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: 710da3c8 ("sched/core: Prevent race condition between cpuset and __sched_setscheduler()")
Reported-by: default avatarMike Galbraith <efault@gmx.de>
Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/a23a826af7c108ea5651e73b8fbae5e653f16e86.camel@gmx.de
parent c597bfdd

kernel/kthread.c

+8 −8
Original line number Diff line number Diff line
@@ -270,6 +270,7 @@ 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;
@@ -300,6 +301,13 @@ 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;
@@ -397,7 +405,6 @@ 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];



		/*
@@ -406,13 +413,6 @@ 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;