From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Thu, 21 Mar 2013 19:01:05 +0100
Subject: [PATCH 208/351] printk: Drop the logbuf_lock more often
Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=a7251c41471c363cfc01a070c987ce820b30cc17

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 | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 273f2c7864b5..1e0988a0bb97 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1457,12 +1457,23 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 	u64 next_seq;
 	u64 seq;
 	u32 idx;
+	int attempts = 0;
+	int num_msg;
 
 	text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
 	if (!text)
 		return -ENOMEM;
 
 	logbuf_lock_irq();
+
+try_again:
+	attempts++;
+	if (attempts > 10) {
+		len = -EBUSY;
+		goto out;
+	}
+	num_msg = 0;
+
 	/*
 	 * Find first record that fits, including all following records,
 	 * into the user-provided buffer for this dump.
@@ -1475,6 +1486,14 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 		len += msg_print_text(msg, true, NULL, 0);
 		idx = log_next(idx);
 		seq++;
+		num_msg++;
+		if (num_msg > 5) {
+			num_msg = 0;
+			logbuf_unlock_irq();
+			logbuf_lock_irq();
+			if (clear_seq < log_first_seq)
+				goto try_again;
+		}
 	}
 
 	/* move first record forward until length fits into the buffer */
@@ -1486,6 +1505,14 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 		len -= msg_print_text(msg, true, NULL, 0);
 		idx = log_next(idx);
 		seq++;
+		num_msg++;
+		if (num_msg > 5) {
+			num_msg = 0;
+			logbuf_unlock_irq();
+			logbuf_lock_irq();
+			if (clear_seq < log_first_seq)
+				goto try_again;
+		}
 	}
 
 	/* last message fitting into this dump */
@@ -1523,6 +1550,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 		clear_seq = log_next_seq;
 		clear_idx = log_next_idx;
 	}
+out:
 	logbuf_unlock_irq();
 
 	kfree(text);