kernel/watchdog.c: avoid races between /proc handlers and CPU hotplug

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>
Acked-by: Don Zickus <dzickus@redhat.com>
Reviewed-by: Aaron Tomlin <atomlin@redhat.com>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 7357842da933..13fdda1a4c91 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;
 }