Re: [PATCH v2 1/2] hung_task: Show the blocker task if the task is hung on mutex

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Feb 21, 2025 at 11:30:01PM +0900, Masami Hiramatsu (Google) wrote:
> +static void debug_show_blocker(struct task_struct *task)
> +{
> +	struct task_struct *g, *t;
> +	unsigned long owner;
> +	struct mutex *lock;
> +
> +	lock = READ_ONCE(task->blocker_mutex);
> +	if (!lock)
> +		return;
> +
> +	owner = mutex_get_owner(lock);
> +	if (likely(owner)) {
> +		/* Ensure the owner information is correct. */
> +		for_each_process_thread(g, t)
> +			if ((unsigned long)t == owner) {
> +				pr_err("INFO: task %s:%d is blocked on a mutex owned by task %s:%d.\n",
> +					task->comm, task->pid, t->comm, t->pid);
> +				sched_show_task(t);
> +				return;
> +			}

 - that for_each_process_thread() scope needs { }

 - that for_each_process_thread() loop needs RCU or tasklist_lock

 - there is no saying that the owner you read with mutex_get_owner() is
   still the owner by the time you do the compare, there can have been
   owner changes.

> +	}
> +	pr_err("INFO: task %s:%d is blocked on a mutex, but the owner is not found.\n",
> +		task->comm, task->pid);
> +}