From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Thu, 21 Mar 2013 19:01:05 +0100
Subject: printk: Drop the logbuf_lock more often
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: 1c49dfbd4a11ce4db947e5999aa0089f029e4f14
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

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>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 kernel/printk/printk.c |   27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1422,6 +1422,8 @@ static int syslog_print_all(char __user
 	char *text;
 	bool time;
 	int len = 0;
+	int attempts = 0;
+	int num_msg;
 
 	text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
 	if (!text)
@@ -1434,6 +1436,14 @@ static int syslog_print_all(char __user
 		u64 seq;
 		u32 idx;
 
+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.
@@ -1446,6 +1456,14 @@ static int syslog_print_all(char __user
 			len += msg_print_text(msg, true, time, 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 */
@@ -1457,6 +1475,14 @@ static int syslog_print_all(char __user
 			len -= msg_print_text(msg, true, time, 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 */
@@ -1490,6 +1516,7 @@ static int syslog_print_all(char __user
 		clear_seq = log_next_seq;
 		clear_idx = log_next_idx;
 	}
+out:
 	logbuf_unlock_irq();
 
 	kfree(text);