Re: [PATCH v3] hung_task: Explicitly report I/O wait state in log output

[Petr Mladek]

On Sat 2026-02-07 19:24:58, 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 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.

IMHO, this is not true. The blocked tasks might wake up at any time.
There is a small chance to print an inconsistent information.
But we could live with it. The entire hung task report is racy.

The race would actually be a lucky moment where the task get
unblocked. In this case, it won't be reported in the next
round...

I suggest to omit the entire paragraph.

> Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>

Othrewise, it looks good.

With the removed paragraph:

Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>

Best Regards,
Petr