[RFC][PATCHv2 7/8] printk: add printk emergency_mode parameter

[Sergey Senozhatsky]

This param permits user-space to forcibly on/off printk emergency
mode via /sys/module/printk/parameters/emergency_mode node.

We have annotated sections in the kernel that switch printk to
emergency, but there might be places/cases when user space would
want to have printk operate in emergency mode all the time.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
 kernel/printk/printk.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1927b5cb5cbe..0d96839bb450 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -455,7 +455,7 @@ static struct task_struct *printk_kthread __read_mostly;
 static atomic_t printk_emergency __read_mostly;
 /*
  * Disable printk_kthread permanently. Unlike `oops_in_progress'
- * it doesn't go back to 0.
+ * it doesn't go back to 0 (unless enforced by user-space).
  */
 static bool printk_kthread_disabled __read_mostly;
 
@@ -483,6 +483,24 @@ void printk_emergency_end(void)
 	atomic_dec(&printk_emergency);
 }
 
+static int printk_kthread_disabled_set(const char *val,
+					const struct kernel_param *kp)
+{
+	return param_set_bool(val, kp);
+}
+
+static const struct kernel_param_ops printk_kthread_disabled_ops = {
+	.set = printk_kthread_disabled_set,
+	.get = param_get_bool,
+};
+
+module_param_cb(emergency_mode,
+		&printk_kthread_disabled_ops,
+		&printk_kthread_disabled,
+		0644);
+MODULE_PARM_DESC(emergency_mode,
+		"don't offload message printing to printk kthread");
+
 /* Return log buffer address */
 char *log_buf_addr_get(void)
 {
-- 
2.12.2