kdb: Implement disable_nmi command

This command disables NMI-entry. If NMI source has been previously shared
with a serial console ("debug port"), this effectively releases the port
from KDB exclusive use, and makes the console available for normal use.

Of course, NMI can be reenabled, enable_nmi modparam is used for that:

	echo 1 > /sys/module/kdb/parameters/enable_nmi

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Acked-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 31 insertions, 0 deletions

diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index 31df1706b9a9..1261dc7eaeb9 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -21,6 +21,7 @@
 #include <linux/smp.h>
 #include <linux/utsname.h>
 #include <linux/vmalloc.h>
+#include <linux/atomic.h>
 #include <linux/module.h>
 #include <linux/mm.h>
 #include <linux/init.h>
@@ -2107,6 +2108,32 @@ static int kdb_dmesg(int argc, const char **argv)
         return 0;
 }
 #endif /* CONFIG_PRINTK */
+
+/* Make sure we balance enable/disable calls, must disable first. */
+static atomic_t kdb_nmi_disabled;
+
+static int kdb_disable_nmi(int argc, const char *argv[])
+{
+        if (atomic_read(&kdb_nmi_disabled))
+                return 0;
+        atomic_set(&kdb_nmi_disabled, 1);
+        arch_kgdb_ops.enable_nmi(0);
+        return 0;
+}
+
+static int kdb_param_enable_nmi(const char *val, const struct kernel_param *kp)
+{
+        if (!atomic_add_unless(&kdb_nmi_disabled, -1, 0))
+                return -EINVAL;
+        arch_kgdb_ops.enable_nmi(1);
+        return 0;
+}
+
+static const struct kernel_param_ops kdb_param_ops_enable_nmi = {
+        .set = kdb_param_enable_nmi,
+};
+module_param_cb(enable_nmi, &kdb_param_ops_enable_nmi, NULL, 0600);
+
 /*
  * kdb_cpu - This function implements the 'cpu' command.
  *      cpu     [<cpunum>]
@@ -2851,6 +2878,10 @@ static void __init kdb_inittab(void)
         kdb_register_repeat("dmesg", kdb_dmesg, "[lines]",
           "Display syslog buffer", 0, KDB_REPEAT_NONE);
 #endif
+        if (arch_kgdb_ops.enable_nmi) {
+                kdb_register_repeat("disable_nmi", kdb_disable_nmi, "",
+                  "Disable NMI entry to KDB", 0, KDB_REPEAT_NONE);
+        }
         kdb_register_repeat("defcmd", kdb_defcmd, "name \"usage\" \"help\"",
           "Define a set of commands, down to endefcmd", 0, KDB_REPEAT_NONE);
         kdb_register_repeat("kill", kdb_kill, "<-signal> <pid>",