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

[Peter Zijlstra]

On Tue, Nov 10, 2020 at 12:38:47PM +0100, Sebastian Andrzej Siewior wrote:
> 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@xxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Link: https://lkml.kernel.org/r/a23a826af7c108ea5651e73b8fbae5e653f16e86.camel@xxxxxx

Moo... yes this is certainly the easiest solution, because nouveau is a
horrible rats nest. But when I spoke to Greg KH about this, he suggested
nouveau ought to be fixed.

Ben, I got terminally lost when trying to untangle nouvea init, is there
any chance this can be fixed to not hold that nvkm_device::mutex thing
while doing request_irq() ?

> ---
>  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
>