From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: printk: make printk.synchronous param rw
Patch-mainline: Not yet, still discussed upstream
References: bnc#744692, bnc#789311, bsc#1174146

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).

[pmladek: Introduce can_printk_async() that will be used later.
	Do not introduce userspace interface that will not be
	supported upstream.
]

Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
---
 kernel/printk/printk.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -461,6 +461,11 @@ static struct task_struct *printk_kthrea
 /* When `true' printing thread has messages to print */
 static bool printk_kthread_need_flush_console;
 
+static inline bool can_printk_async(void)
+{
+	return printk_kthread;
+}
+
 /*
  * We cannot access per-CPU data (e.g. per-CPU flush irq_work) before
  * per_cpu_areas are initialised. This variable is set to true when
@@ -2073,7 +2078,7 @@ asmlinkage int vprintk_emit(int facility
 		 * 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);
@@ -3141,7 +3146,7 @@ static void wake_up_klogd_work_func(stru
 	int pending = this_cpu_xchg(printk_pending, 0);
 
 	if (pending & PRINTK_PENDING_OUTPUT) {
-		if (printk_kthread) {
+		if (can_printk_async()) {
 			/* Offload printing to a schedulable context. */
 			printk_kthread_need_flush_console = true;
 			wake_up_process(printk_kthread);