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@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/n/tip-j8swthl12u73h4znbvitljzd@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 5 insertions, 3 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 35185392173f..751e7b84e9e3 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();
 }