[PATCH] printk: make printk.synchronous param rw

[Sergey Senozhatsky]

Change `synchronous' printk param to be RW, so user space
can change printk mode back and forth to/from sync mode
(which is considered to be more reliable).

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
---

-- added Jan's Reviewed-by
-- factored out async printk checks to can_printk_async()

 kernel/printk/printk.c | 56 ++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 45 insertions(+), 11 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 89f5441..9345a29 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -288,14 +288,16 @@ static u32 log_buf_len = __LOG_BUF_LEN;
 
 /* Control whether printing to console must be synchronous. */
 static bool __read_mostly printk_sync = true;
-module_param_named(synchronous, printk_sync, bool, S_IRUGO);
-MODULE_PARM_DESC(synchronous, "make printing to console synchronous");
-
 /* Printing kthread for async printk */
 static struct task_struct *printk_kthread;
 /* When `true' printing thread has messages to print */
 static bool printk_kthread_need_flush_console;
 
+static inline bool can_printk_async(void)
+{
+	return !printk_sync && printk_kthread;
+}
+
 /* Return log buffer address */
 char *log_buf_addr_get(void)
 {
@@ -1785,7 +1787,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 		 * operate in sync mode once panic() occurred.
 		 */
 		if (console_loglevel != CONSOLE_LOGLEVEL_MOTORMOUTH &&
-				printk_kthread) {
+				can_printk_async()) {
 			/* Offload printing to a schedulable context. */
 			printk_kthread_need_flush_console = true;
 			wake_up_process(printk_kthread);
@@ -2757,6 +2759,13 @@ static int __init printk_late_init(void)
 late_initcall(printk_late_init);
 
 #if defined CONFIG_PRINTK
+/*
+ * Prevent starting printk_kthread from start_kernel()->parse_args().
+ * It's not possible at this stage. Instead, do it via the inticall
+ * or a sysfs knob.
+ */
+static bool printk_kthread_can_run;
+
 static int printk_kthread_func(void *data)
 {
 	while (1) {
@@ -2780,18 +2789,14 @@ static int printk_kthread_func(void *data)
 	return 0;
 }
 
-/*
- * Init async printk via late_initcall, after core/arch/device/etc.
- * initialization.
- */
-static int __init init_printk_kthread(void)
+static int __init_printk_kthread(void)
 {
 	struct task_struct *thread;
 	struct sched_param param = {
 		.sched_priority = MAX_RT_PRIO - 1,
 	};
 
-	if (printk_sync)
+	if (!printk_kthread_can_run || printk_sync || printk_kthread)
 		return 0;
 
 	thread = kthread_run(printk_kthread_func, NULL, "printk");
@@ -2805,6 +2810,35 @@ static int __init init_printk_kthread(void)
 	printk_kthread = thread;
 	return 0;
 }
+
+static int printk_sync_set(const char *val, const struct kernel_param *kp)
+{
+	int ret;
+
+	ret = param_set_bool(val, kp);
+	if (ret)
+		return ret;
+	return __init_printk_kthread();
+}
+
+static const struct kernel_param_ops param_ops_printk_sync = {
+	.set = printk_sync_set,
+	.get = param_get_bool,
+};
+
+module_param_cb(synchronous, &param_ops_printk_sync, &printk_sync,
+		S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(synchronous, "make printing to console synchronous");
+
+/*
+ * Init async printk via late_initcall, after core/arch/etc.
+ * initialization.
+ */
+static __init int init_printk_kthread(void)
+{
+	printk_kthread_can_run = true;
+	return __init_printk_kthread();
+}
 late_initcall(init_printk_kthread);
 
 /*
@@ -2820,7 +2854,7 @@ static void wake_up_klogd_work_func(struct irq_work *irq_work)
 	int pending = __this_cpu_xchg(printk_pending, 0);
 
 	if (pending & PRINTK_PENDING_OUTPUT) {
-		if (printk_kthread) {
+		if (can_printk_async()) {
 			wake_up_process(printk_kthread);
 		} else {
 			/*
-- 
2.8.0