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Subject[PATCH] printk: make printk.synchronous param rw
Date
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@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
---

-- 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
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