Re: [v2 PATCH 1/1] hung_task: Explicitly report I/O wait state in log output

[Lance Yang]

On 2026/2/2 23:14, Petr Mladek wrote:
> On Wed 2026-01-28 15:45:16, Aaron Tomlin wrote:
> > Currently, the hung task reporting mechanism indiscriminately labels all
> > TASK_UNINTERRUPTIBLE (D) tasks as "blocked", irrespective of whether they
> > are awaiting I/O completion or kernel locking primitives. This ambiguity
> > compels system administrators to manually inspect stack traces to discern
> > whether the delay stems from an I/O wait (typically indicative of
> > hardware or filesystem anomalies) or software contention. Such detailed
> > analysis is not always immediately accessible to system administrators
> > or support engineers.
> >
> > To address this, this patch utilises the existing in_iowait field within
> > struct task_struct to augment the failure report. If the task is blocked
> > due to I/O (e.g., via io_schedule_prepare()), the log message is updated
> > to explicitly state "blocked in I/O wait".
> >
> > Examples:
> >          - Standard Block: "INFO: task bash:123 blocked for more than 120
> >            seconds".
> >
> >          - I/O Block: "INFO: task dd:456 blocked in I/O wait for more than
> >            120 seconds".
> >
> > Accessing in_iowait is safe in this context. The detector holds
> > rcu_read_lock() within check_hung_uninterruptible_tasks(), ensuring the
> > task structure remains valid in memory.
>
> This part is true.
>
> > Furthermore, as the task is
> > confirmed to be in a persistent TASK_UNINTERRUPTIBLE state, it cannot
> > modify its own in_iowait flag, rendering the read operation stable and
> > free from data races.
>
> Strictly speaking, this is not true. IMHO, nothing prevents the task
> from waking up in parallel. Just the chance is small.
>
> I would say that the information will be valid in 99.99% of situations
> because the message is printed only when the task has been stuck
> in the state for a long time. A possible mistake should be
> visible from the later printed backtrace.
>
> > --- a/kernel/hung_task.c
> > +++ b/kernel/hung_task.c
> > @@ -250,8 +250,9 @@ static void hung_task_info(struct task_struct *t, unsigned long timeout,
> >   	if (sysctl_hung_task_warnings || hung_task_call_panic) {
> >   		if (sysctl_hung_task_warnings > 0)
> >   			sysctl_hung_task_warnings--;
> > -		pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
> > -		       t->comm, t->pid, (jiffies - t->last_switch_time) / HZ);
> > +		pr_err("INFO: task %s:%d blocked %s for more than %ld seconds.\n",
>
> s/blocked %s for/blocked%s for/
>
> > +		       t->comm, t->pid, t->in_iowait ? "in I/O wait" : "",
>
> and here: " in I/O wait".
>
> Otherwise, it would print two spaces in the non-io case, like"
>
>    "INFO: task bash:123 blocked  for more than 120 seconds"

Good catch!

> > +		       (jiffies - t->last_switch_time) / HZ);
> >   		pr_err("      %s %s %.*s\n",
> >   			print_tainted(), init_utsname()->release,
> >   			(int)strcspn(init_utsname()->version, " "),
>
> Otherwise, it looks good.

Other than that, looks good to me.