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);
>