[tip:core/printk] printk: Fix console_sem vs logbuf_lock unlock race

[tip-bot for Peter Zijlstra <peterz@infradead.org>]

Commit-ID:  4f2a8d3cf5e0486fd547633fa86c5d130ae98cad
Gitweb:     http://git.kernel.org/tip/4f2a8d3cf5e0486fd547633fa86c5d130ae98cad
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 22 Jun 2011 11:20:09 +0200
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 22 Jun 2011 11:39:34 +0200

printk: Fix console_sem vs logbuf_lock unlock race

Fix up the fallout from commit 0b5e1c5255 ("printk: Release
console_sem after logbuf_lock").

The reason for unlocking the console_sem under the logbuf_lock
is that a concurrent printk() might fill up the buffer but fail
to acquire the console sem, resulting in a missed write to the
console until a subsequent console_sem acquire/release cycle.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: efault@gmx.de
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1308734409.1022.14.camel@twins
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/printk.c |   20 ++++++++++++++++++--
 1 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/printk.c b/kernel/printk.c
index 751e7b8..37dff34 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1244,7 +1244,7 @@ void console_unlock(void)
 {
 	unsigned long flags;
 	unsigned _con_start, _log_end;
-	unsigned wake_klogd = 0;
+	unsigned wake_klogd = 0, retry = 0;
 
 	if (console_suspended) {
 		up(&console_sem);
@@ -1253,6 +1253,7 @@ void console_unlock(void)
 
 	console_may_schedule = 0;
 
+again:
 	for ( ; ; ) {
 		spin_lock_irqsave(&logbuf_lock, flags);
 		wake_klogd |= log_start - log_end;
@@ -1273,8 +1274,23 @@ void console_unlock(void)
 	if (unlikely(exclusive_console))
 		exclusive_console = NULL;
 
-	spin_unlock_irqrestore(&logbuf_lock, flags);
+	spin_unlock(&logbuf_lock);
+
 	up(&console_sem);
+
+	/*
+	 * Someone could have filled up the buffer again, so re-check if there's
+	 * something to flush. In case we cannot trylock the console_sem again,
+	 * there's a new owner and the console_unlock() from them will do the
+	 * flush, no worries.
+	 */
+	spin_lock(&logbuf_lock);
+	if (con_start != log_end)
+		retry = 1;
+	spin_unlock_irqrestore(&logbuf_lock, flags);
+	if (retry && console_trylock())
+		goto again;
+
 	if (wake_klogd)
 		wake_up_klogd();
 }