Re: [RFC PATCH 11/20] kthread: Make sure kthread hasn't started while binding it

[Vlastimil Babka <vbabka@suse.cz>]

On 7/26/24 11:56 PM, Frederic Weisbecker wrote:
> Make sure the kthread is sleeping in the schedule_preempt_disabled()
> call before calling its handler when kthread_bind[_mask]() is called
> on it. This provides a sanity check verifying that the task is not
> randomly blocked later at some point within its function handler, in
> which case it could be just concurrently awaken, leaving the call to
> do_set_cpus_allowed() without any effect until the next voluntary sleep.
> 
> Rely on the wake-up ordering to ensure that the newly introduced "started"
> field returns the expected value:
> 
>     TASK A                                   TASK B
>     ------                                   ------
> READ kthread->started
> wake_up_process(B)
>    rq_lock()
>    ...
>    rq_unlock() // RELEASE
>                                            schedule()
>                                               rq_lock() // ACQUIRE
>                                               // schedule task B
>                                               rq_unlock()
>                                               WRITE kthread->started
> 
> Similarly, writing kthread->started before subsequent voluntary sleeps
> will be visible after calling wait_task_inactive() in
> __kthread_bind_mask(), reporting potential misuse of the API.
> 
> Upcoming patches will make further use of this facility.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  kernel/kthread.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index f7be976ff88a..ecb719f54f7a 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -53,6 +53,7 @@ struct kthread_create_info
>  struct kthread {
>  	unsigned long flags;
>  	unsigned int cpu;
> +	int started;
>  	int result;
>  	int (*threadfn)(void *);
>  	void *data;
> @@ -382,6 +383,8 @@ static int kthread(void *_create)
>  	schedule_preempt_disabled();
>  	preempt_enable();
>  
> +	self->started = 1;
> +
>  	ret = -EINTR;
>  	if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
>  		cgroup_kthread_ready();
> @@ -540,7 +543,9 @@ static void __kthread_bind(struct task_struct *p, unsigned int cpu, unsigned int
>  
>  void kthread_bind_mask(struct task_struct *p, const struct cpumask *mask)
>  {
> +	struct kthread *kthread = to_kthread(p);
>  	__kthread_bind_mask(p, mask, TASK_UNINTERRUPTIBLE);
> +	WARN_ON_ONCE(kthread->started);
>  }
>  
>  /**
> @@ -554,7 +559,9 @@ void kthread_bind_mask(struct task_struct *p, const struct cpumask *mask)
>   */
>  void kthread_bind(struct task_struct *p, unsigned int cpu)
>  {
> +	struct kthread *kthread = to_kthread(p);
>  	__kthread_bind(p, cpu, TASK_UNINTERRUPTIBLE);
> +	WARN_ON_ONCE(kthread->started);
>  }
>  EXPORT_SYMBOL(kthread_bind);
>