[PATCH v2 2/2] hung_task: Enable runtime reset of hung_task_detect_count

[Aaron Tomlin <atomlin@atomlin.com>]

Introduce support for writing to /proc/sys/kernel/hung_task_detect_count.

Writing any value to this file atomically resets the counter of detected
hung tasks to zero. This grants system administrators the ability to clear
the cumulative diagnostic history after resolving an incident, simplifying
monitoring without requiring a system restart.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 Documentation/admin-guide/sysctl/kernel.rst |  2 +-
 kernel/hung_task.c                          | 30 +++++++++++++++++++--
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 239da22c4e28..43c17b919969 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -418,7 +418,7 @@ hung_task_detect_count
 ======================
 
 Indicates the total number of tasks that have been detected as hung since
-the system boot.
+the system boot. The counter can be reset to zero when written to.
 
 This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
 
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 6f3fb26378b5..979b7e2fcc19 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -377,6 +377,32 @@ static long hung_timeout_jiffies(unsigned long last_checked,
 }
 
 #ifdef CONFIG_SYSCTL
+
+/**
+ * proc_dohung_task_detect_count - proc handler for hung_task_detect_count
+ *
+ * Handles read access for the hung task counter. For write access, it
+ * accepts any successfully parsed value and atomically resets the counter
+ * to zero. Returns the byte count written on success or a negative error
+ * code on failure.
+ */
+static int proc_dohung_task_detect_count(const struct ctl_table *table, int write,
+					 void *buffer, size_t *lenp, loff_t *ppos)
+{
+	int ret;
+
+	if (!write)
+		return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
+
+	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
+	if (ret)
+		return ret;
+
+	WRITE_ONCE(sysctl_hung_task_detect_count, 0);
+
+	return ret;
+}
+
 /*
  * Process updating of timeout sysctl
  */
@@ -459,8 +485,8 @@ static const struct ctl_table hung_task_sysctls[] = {
 		.procname	= "hung_task_detect_count",
 		.data		= &sysctl_hung_task_detect_count,
 		.maxlen		= sizeof(unsigned long),
-		.mode		= 0444,
-		.proc_handler	= proc_doulongvec_minmax,
+		.mode		= 0644,
+		.proc_handler	= proc_dohung_task_detect_count,
 	},
 	{
 		.procname	= "hung_task_sys_info",
-- 
2.51.0