[PATCH 2/4] watchdog: avoid races between /proc handlers and CPU hotplug

[Ulrich Obergfell <uobergfe@redhat.com>]

The handler functions for watchdog parameters in /proc/sys/kernel
do not protect themselves against races with CPU hotplug. Hence,
theoretically it is possible that a new watchdog thread is started
on a hotplugged CPU while a parameter is being modified, and the
thread could thus use a parameter value that is 'in transition'.

For example, if 'watchdog_thresh' is being set to zero (note: this
disables the lockup detectors) the thread would erroneously use the
value zero as the sample period.

To avoid such races and to keep the /proc handler code consistent,
call
     {get|put}_online_cpus() in proc_watchdog_common()
     {get|put}_online_cpus() in proc_watchdog_thresh()
     {get|put}_online_cpus() in proc_watchdog_cpumask()

Signed-off-by: Ulrich Obergfell <uobergfe@redhat.com>
---
 kernel/watchdog.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 7357842..13fdda1 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -857,6 +857,7 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
 	int err, old, new;
 	int *watchdog_param = (int *)table->data;
 
+	get_online_cpus();
 	mutex_lock(&watchdog_proc_mutex);
 
 	if (watchdog_suspended) {
@@ -908,6 +909,7 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
 	}
 out:
 	mutex_unlock(&watchdog_proc_mutex);
+	put_online_cpus();
 	return err;
 }
 
@@ -949,6 +951,7 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
 {
 	int err, old;
 
+	get_online_cpus();
 	mutex_lock(&watchdog_proc_mutex);
 
 	if (watchdog_suspended) {
@@ -974,6 +977,7 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
 	}
 out:
 	mutex_unlock(&watchdog_proc_mutex);
+	put_online_cpus();
 	return err;
 }
 
@@ -988,6 +992,7 @@ int proc_watchdog_cpumask(struct ctl_table *table, int write,
 {
 	int err;
 
+	get_online_cpus();
 	mutex_lock(&watchdog_proc_mutex);
 
 	if (watchdog_suspended) {
@@ -1015,6 +1020,7 @@ int proc_watchdog_cpumask(struct ctl_table *table, int write,
 	}
 out:
 	mutex_unlock(&watchdog_proc_mutex);
+	put_online_cpus();
 	return err;
 }
 
-- 
1.7.11.7