hung_task: refactor detection logic and atomicise detection count

hung_task: refactor detection logic and atomicise detection count

Patch series "hung_task: Provide runtime reset interface for hung task
detector", v9.

This series introduces the ability to reset
/proc/sys/kernel/hung_task_detect_count.

Writing a "0" value to this file atomically resets the counter of detected
hung tasks.  This functionality provides system administrators with the
means to clear the cumulative diagnostic history following incident
resolution, thereby simplifying subsequent monitoring without
necessitating a system restart.

This patch (of 3):

The check_hung_task() function currently conflates two distinct
responsibilities: validating whether a task is hung and handling the
subsequent reporting (printing warnings, triggering panics, or
tracepoints).

This patch refactors the logic by introducing hung_task_info(), a function
dedicated solely to reporting.  The actual detection check,
task_is_hung(), is hoisted into the primary loop within
check_hung_uninterruptible_tasks().  This separation clearly decouples the
mechanism of detection from the policy of reporting.

Furthermore, to facilitate future support for concurrent hung task
detection, the global sysctl_hung_task_detect_count variable is converted
from unsigned long to atomic_long_t.  Consequently, the counting logic is
updated to accumulate the number of hung tasks locally (this_round_count)
during the iteration.  The global counter is then updated atomically via
atomic_long_cmpxchg_relaxed() once the loop concludes, rather than
incrementally during the scan.

These changes are strictly preparatory and introduce no functional change
to the system's runtime behaviour.

Link: https://lkml.kernel.org/r/20260303203031.4097316-1-atomlin@atomlin.com
Link: https://lkml.kernel.org/r/20260303203031.4097316-2-atomlin@atomlin.com
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Reviewed-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>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Joel Granados <joel.granados@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>