hung_task: explicitly report I/O wait state in log output

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".

Theoretically, concurrent executions of io_schedule_finish() could result
in a race condition where the read flag does not precisely correlate with
the subsequently printed backtrace.  However, this limitation is deemed
acceptable in practice.  The entire reporting mechanism is inherently racy
by design; nevertheless, it remains highly reliable in the vast majority
of cases, particularly because it primarily captures protracted stalls. 
Consequently, introducing additional synchronisation to mitigate this
minor inaccuracy would be entirely disproportionate to the situation.

Link: https://lkml.kernel.org/r/20260303221324.4106917-1-atomlin@atomlin.com
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 8bc043fbe89c..6fcc94ce4ca9 100644
--- 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",
+		       t->comm, t->pid, t->in_iowait ? " in I/O wait" : "",
+		       (jiffies - t->last_switch_time) / HZ);
 		pr_err("      %s %s %.*s\n",
 			print_tainted(), init_utsname()->release,
 			(int)strcspn(init_utsname()->version, " "),