tty: fix BKL related leak and crash

Enabling the BKL to be lockdep tracked uncovered the following
upstream kernel bug in the tty code, which caused a BKL
reference leak:

  ================================================
  [ BUG: lock held when returning to user space! ]
  ------------------------------------------------
  dmesg/3121 is leaving the kernel with locks still held!
  1 lock held by dmesg/3121:
   #0:  (kernel_mutex){--..}, at: [<c02f34d9>] opost+0x24/0x194

this might explain some of the atomicity warnings and crashes
that -tip tree testing has been experiencing since the BKL
was converted back to a spinlock.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 3 deletions

diff --git a/drivers/char/n_tty.c b/drivers/char/n_tty.c
index 19105ec203f..8096389b0dc 100644
--- a/drivers/char/n_tty.c
+++ b/drivers/char/n_tty.c
@@ -282,16 +282,20 @@ static int opost(unsigned char c, struct tty_struct *tty)
                         if (O_ONLRET(tty))
                                 tty->column = 0;
                         if (O_ONLCR(tty)) {
-                                if (space < 2)
+                                if (space < 2) {
+                                        unlock_kernel();
                                         return -1;
+                                }
                                 tty_put_char(tty, '\r');
                                 tty->column = 0;
                         }
                         tty->canon_column = tty->column;
                         break;
                 case '\r':
-                        if (O_ONOCR(tty) && tty->column == 0)
+                        if (O_ONOCR(tty) && tty->column == 0) {
+                                unlock_kernel();
                                 return 0;
+                        }
                         if (O_OCRNL(tty)) {
                                 c = '\n';
                                 if (O_ONLRET(tty))
@@ -303,10 +307,13 @@ static int opost(unsigned char c, struct tty_struct *tty)
                 case '\t':
                         spaces = 8 - (tty->column & 7);
                         if (O_TABDLY(tty) == XTABS) {
-                                if (space < spaces)
+                                if (space < spaces) {
+                                        unlock_kernel();
                                         return -1;
+                                }
                                 tty->column += spaces;
                                 tty->ops->write(tty, "        ", spaces);
+                                unlock_kernel();
                                 return 0;
                         }
                         tty->column += spaces;