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

[Waiman Long]

On 2/21/25 1:59 PM, Peter Zijlstra wrote:
> 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

The call chain should be

check_hung_uninterruptible_tasks()
  -> check_hung_task()
    -> debug_show_blocker()

check_hung_uninterruptible_tasks() takes rcu_read_lock() before calling 
check_hung_task(). Perhaps add a statement like

    RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "no rcu lock held");

>   - 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.

Maybe change "owned by" to "likely owned by" :-)

Cheers,
Longman