[tip:core/printk] printk: Release console_sem after logbuf_lock

[tip-bot for Peter Zijlstra]

Commit-ID:  0b5e1c5255e7ee8670e077e8224e5c2281229a5b
Gitweb:     http://git.kernel.org/tip/0b5e1c5255e7ee8670e077e8224e5c2281229a5b
Author:     Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
AuthorDate: Tue, 7 Jun 2011 11:15:33 +0200
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Tue, 7 Jun 2011 12:50:02 +0200

printk: Release console_sem after logbuf_lock

Release console_sem after unlocking the logbuf_lock so that we don't
generate wakeups while holding logbuf_lock. This avoids some lock
inversion troubles once we remove the lockdep_off bits between
logbuf_lock and rq->lock (prints while holding rq->lock vs doing
wakeups while holding logbuf_lock).

There's of course still an actual deadlock where the printk()s under
rq->lock will issue a wakeup from the up() call, but lockdep won't
warn about that since semaphores are not tracked.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/n/tip-j8swthl12u73h4znbvitljzd@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 kernel/printk.c |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/printk.c b/kernel/printk.c
index 3518539..751e7b8 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -782,7 +782,7 @@ static inline int can_use_console(unsigned int cpu)
 static int console_trylock_for_printk(unsigned int cpu)
 	__releases(&logbuf_lock)
 {
-	int retval = 0;
+	int retval = 0, wake = 0;
 
 	if (console_trylock()) {
 		retval = 1;
@@ -795,12 +795,14 @@ static int console_trylock_for_printk(unsigned int cpu)
 		 */
 		if (!can_use_console(cpu)) {
 			console_locked = 0;
-			up(&console_sem);
+			wake = 1;
 			retval = 0;
 		}
 	}
 	printk_cpu = UINT_MAX;
 	spin_unlock(&logbuf_lock);
+	if (wake)
+		up(&console_sem);
 	return retval;
 }
 static const char recursion_bug_msg [] =
@@ -1271,8 +1273,8 @@ void console_unlock(void)
 	if (unlikely(exclusive_console))
 		exclusive_console = NULL;
 
-	up(&console_sem);
 	spin_unlock_irqrestore(&logbuf_lock, flags);
+	up(&console_sem);
 	if (wake_klogd)
 		wake_up_klogd();
 }
--
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