From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Thu, 21 Mar 2013 19:01:05 +0100
Subject: printk: Drop the logbuf_lock more often
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.6/older/patches-4.6.2-rt5.tar.xz

The lock is hold with irgs off. The latency drops 500us+ on my arm bugs
with a "full" buffer after executing "dmesg" on the shell.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/printk/printk.c |   26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1268,6 +1268,7 @@ static int syslog_print_all(char __user
 {
 	char *text;
 	int len = 0;
+	int attempts = 0;
 
 	text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
 	if (!text)
@@ -1279,6 +1280,14 @@ static int syslog_print_all(char __user
 		u64 seq;
 		u32 idx;
 		enum log_flags prev;
+		int num_msg;
+try_again:
+		attempts++;
+		if (attempts > 10) {
+			len = -EBUSY;
+			goto out;
+		}
+		num_msg = 0;
 
 		/*
 		 * Find first record that fits, including all following records,
@@ -1294,6 +1303,14 @@ static int syslog_print_all(char __user
 			prev = msg->flags;
 			idx = log_next(idx);
 			seq++;
+			num_msg++;
+			if (num_msg > 5) {
+				num_msg = 0;
+				raw_spin_unlock_irq(&logbuf_lock);
+				raw_spin_lock_irq(&logbuf_lock);
+				if (clear_seq < log_first_seq)
+					goto try_again;
+			}
 		}
 
 		/* move first record forward until length fits into the buffer */
@@ -1307,6 +1324,14 @@ static int syslog_print_all(char __user
 			prev = msg->flags;
 			idx = log_next(idx);
 			seq++;
+			num_msg++;
+			if (num_msg > 5) {
+				num_msg = 0;
+				raw_spin_unlock_irq(&logbuf_lock);
+				raw_spin_lock_irq(&logbuf_lock);
+				if (clear_seq < log_first_seq)
+					goto try_again;
+			}
 		}
 
 		/* last message fitting into this dump */
@@ -1347,6 +1372,7 @@ static int syslog_print_all(char __user
 		clear_seq = log_next_seq;
 		clear_idx = log_next_idx;
 	}
+out:
 	raw_spin_unlock_irq(&logbuf_lock);
 
 	kfree(text);
