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

tip-bot for Peter Zijlstra
2011-06-22T14:22:44Z

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();
}