linux-kernel - [PATCHv2] printk: wake up klogd in vprintk

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Date:   Thu, 19 Apr 2018 10:42:50 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: [PATCHv2] printk: wake up klogd in vprintk_emit

We wake up klogd very late - only when current console_sem owner
is done pushing pending kernel messages to the serial/net consoles.
In some cases this results in lost syslog messages, because kernel
log buffer is a circular buffer and if we don't wakeup syslog long
enough there are chances that logbuf simply will wrap around.

The patch moves the klog wake up call to vprintk_emit(), which is
the only legit way for a kernel message to appear in the logbuf,
right before we attempt to grab the console_sem (possibly spinning
on it waiting for the hand off) and call console drivers.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
 kernel/printk/printk.c | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2f4af216bd6e..247808333ba4 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1908,6 +1908,7 @@ asmlinkage int vprintk_emit(int facility, int level,
 		preempt_enable();
 	}
 
+	wake_up_klogd();
 	return printed_len;
 }
 EXPORT_SYMBOL(vprintk_emit);
@@ -2289,9 +2290,7 @@ void console_unlock(void)
 {
 	static char ext_text[CONSOLE_EXT_LOG_MAX];
 	static char text[LOG_LINE_MAX + PREFIX_MAX];
-	static u64 seen_seq;
 	unsigned long flags;
-	bool wake_klogd = false;
 	bool do_cond_resched, retry;
 
 	if (console_suspended) {
@@ -2335,11 +2334,6 @@ void console_unlock(void)
 
 		printk_safe_enter_irqsave(flags);
 		raw_spin_lock(&logbuf_lock);
-		if (seen_seq != log_next_seq) {
-			wake_klogd = true;
-			seen_seq = log_next_seq;
-		}
-
 		if (console_seq < log_first_seq) {
 			len = sprintf(text, "** %u printk messages dropped **\n",
 				      (unsigned)(log_first_seq - console_seq));
@@ -2397,7 +2391,7 @@ void console_unlock(void)
 
 		if (console_lock_spinning_disable_and_check()) {
 			printk_safe_exit_irqrestore(flags);
-			goto out;
+			return;
 		}
 
 		printk_safe_exit_irqrestore(flags);
@@ -2429,10 +2423,6 @@ void console_unlock(void)
 
 	if (retry && console_trylock())
 		goto again;
-
-out:
-	if (wake_klogd)
-		wake_up_klogd();
 }
 EXPORT_SYMBOL(console_unlock);
 
-- 
2.17.0