kernel/watchdog.c: control hard lockup detection default

In some cases we don't want hard lockup detection enabled by default.
An example is when running as a guest.  Introduce

  watchdog_enable_hardlockup_detector(bool)

allowing those cases to disable hard lockup detection.  This must be
executed early by the boot processor from e.g.  smp_prepare_boot_cpu, in
order to allow kernel command line arguments to override it, as well as
to avoid hard lockup detection being enabled before we've had a chance
to indicate that it's unwanted.  In summary,

  initial boot:					default=enabled
  smp_prepare_boot_cpu
    watchdog_enable_hardlockup_detector(false):	default=disabled
  cmdline has 'nmi_watchdog=1':			default=enabled

The running kernel still has the ability to enable/disable at any time
with /proc/sys/kernel/nmi_watchdog us usual.  However even when the
default has been overridden /proc/sys/kernel/nmi_watchdog will initially
show '1'.  To truly turn it on one must disable/enable it, i.e.

  echo 0 > /proc/sys/kernel/nmi_watchdog
  echo 1 > /proc/sys/kernel/nmi_watchdog

This patch will be immediately useful for KVM with the next patch of this
series.  Other hypervisor guest types may find it useful as well.

[akpm@linux-foundation.org: fix build]
[dzickus@redhat.com: fix compile issues on sparc]
Signed-off-by: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 48 insertions, 2 deletions

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index ff7fd80bef99..49e9537f3673 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -59,6 +59,25 @@ static unsigned long soft_lockup_nmi_warn;
 static int hardlockup_panic =
                         CONFIG_BOOTPARAM_HARDLOCKUP_PANIC_VALUE;
 
+static bool hardlockup_detector_enabled = true;
+/*
+ * We may not want to enable hard lockup detection by default in all cases,
+ * for example when running the kernel as a guest on a hypervisor. In these
+ * cases this function can be called to disable hard lockup detection. This
+ * function should only be executed once by the boot processor before the
+ * kernel command line parameters are parsed, because otherwise it is not
+ * possible to override this in hardlockup_panic_setup().
+ */
+void watchdog_enable_hardlockup_detector(bool val)
+{
+        hardlockup_detector_enabled = val;
+}
+
+bool watchdog_hardlockup_detector_is_enabled(void)
+{
+        return hardlockup_detector_enabled;
+}
+
 static int __init hardlockup_panic_setup(char *str)
 {
         if (!strncmp(str, "panic", 5))
@@ -67,6 +86,14 @@ static int __init hardlockup_panic_setup(char *str)
                 hardlockup_panic = 0;
         else if (!strncmp(str, "0", 1))
                 watchdog_user_enabled = 0;
+        else if (!strncmp(str, "1", 1) || !strncmp(str, "2", 1)) {
+                /*
+                 * Setting 'nmi_watchdog=1' or 'nmi_watchdog=2' (legacy option)
+                 * has the same effect.
+                 */
+                watchdog_user_enabled = 1;
+                watchdog_enable_hardlockup_detector(true);
+        }
         return 1;
 }
 __setup("nmi_watchdog=", hardlockup_panic_setup);
@@ -465,6 +492,15 @@ static int watchdog_nmi_enable(unsigned int cpu)
         struct perf_event_attr *wd_attr;
         struct perf_event *event = per_cpu(watchdog_ev, cpu);
 
+        /*
+         * Some kernels need to default hard lockup detection to
+         * 'disabled', for example a guest on a hypervisor.
+         */
+        if (!watchdog_hardlockup_detector_is_enabled()) {
+                event = ERR_PTR(-ENOENT);
+                goto handle_err;
+        }
+
         /* is it already setup and enabled? */
         if (event && event->state > PERF_EVENT_STATE_OFF)
                 goto out;
@@ -479,6 +515,7 @@ static int watchdog_nmi_enable(unsigned int cpu)
         /* Try to register using hardware perf events */
         event = perf_event_create_kernel_counter(wd_attr, cpu, NULL, watchdog_overflow_callback, NULL);
 
+handle_err:
         /* save cpu0 error for future comparision */
         if (cpu == 0 && IS_ERR(event))
                 cpu0_err = PTR_ERR(event);
@@ -624,11 +661,13 @@ int proc_dowatchdog(struct ctl_table *table, int write,
                     void __user *buffer, size_t *lenp, loff_t *ppos)
 {
         int err, old_thresh, old_enabled;
+        bool old_hardlockup;
         static DEFINE_MUTEX(watchdog_proc_mutex);
 
         mutex_lock(&watchdog_proc_mutex);
         old_thresh = ACCESS_ONCE(watchdog_thresh);
         old_enabled = ACCESS_ONCE(watchdog_user_enabled);
+        old_hardlockup = watchdog_hardlockup_detector_is_enabled();
 
         err = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
         if (err || !write)
@@ -640,15 +679,22 @@ int proc_dowatchdog(struct ctl_table *table, int write,
          * disabled. The 'watchdog_running' variable check in
          * watchdog_*_all_cpus() function takes care of this.
          */
-        if (watchdog_user_enabled && watchdog_thresh)
+        if (watchdog_user_enabled && watchdog_thresh) {
+                /*
+                 * Prevent a change in watchdog_thresh accidentally overriding
+                 * the enablement of the hardlockup detector.
+                 */
+                if (watchdog_user_enabled != old_enabled)
+                        watchdog_enable_hardlockup_detector(true);
                 err = watchdog_enable_all_cpus(old_thresh != watchdog_thresh);
-        else
+        } else
                 watchdog_disable_all_cpus();
 
         /* Restore old values on failure */
         if (err) {
                 watchdog_thresh = old_thresh;
                 watchdog_user_enabled = old_enabled;
+                watchdog_enable_hardlockup_detector(old_hardlockup);
         }
 out:
         mutex_unlock(&watchdog_proc_mutex);