tty_check_change(): avoid taking tasklist_lock while holding tty->ctrl_lock

May 11 09:42:27 [kernel] [ 1104.496819] rarian-sk-get-c[5630]: segfault at 0 ip 7f478556caf0 sp 7fff8e3fe338 error 4 in libc-2.6.1.so[7f47854f9000+136000]
May 11 10:59:48 [kernel] [ 2494.165792]
May 11 10:59:48 [kernel] [ 2494.165794] =======================================================
May 11 10:59:48 [kernel] [ 2494.165801] [ INFO: possible circular locking dependency detected ]
May 11 10:59:48 [kernel] [ 2494.165805] 2.6.26-rc1-00007-g91b3a7a #217
May 11 10:59:48 [kernel] [ 2494.165807] -------------------------------------------------------
May 11 10:59:48 [kernel] [ 2494.165809] less/7053 is trying to acquire lock:
May 11 10:59:48 [kernel] [ 2494.165812]  (tasklist_lock){..??}, at: [<ffffffff80232e95>] is_current_pgrp_orphaned+0x15/0x50
May 11 10:59:48 [kernel] [ 2494.165821]
May 11 10:59:48 [kernel] [ 2494.165822] but task is already holding lock:
May 11 10:59:48 [kernel] [ 2494.165824]  (&tty->ctrl_lock){....}, at: [<ffffffff803d5f31>] tty_check_change+0x61/0x110
May 11 10:59:48 [kernel] [ 2494.165831]
May 11 10:59:48 [kernel] [ 2494.165832] which lock already depends on the new lock.
May 11 10:59:48 [kernel] [ 2494.165833]
May 11 10:59:48 [kernel] [ 2494.165835]
May 11 10:59:48 [kernel] [ 2494.165836] the existing dependency chain (in reverse order) is:
May 11 10:59:48 [kernel] [ 2494.165838]
May 11 10:59:48 [kernel] [ 2494.165839] -> #2 (&tty->ctrl_lock){....}:
May 11 10:59:48 [kernel] [ 2494.165843]        [<ffffffff80253796>] __lock_acquire+0xf86/0x1080
May 11 10:59:48 [kernel] [ 2494.165851]        [<ffffffff80253922>] lock_acquire+0x92/0xc0
May 11 10:59:48 [kernel] [ 2494.165858]        [<ffffffff804deee0>] _spin_lock_irqsave+0x40/0x60
May 11 10:59:48 [kernel] [ 2494.165866]        [<ffffffff803d31b5>] __proc_set_tty+0x35/0xe0
May 11 10:59:48 [kernel] [ 2494.165873]        [<ffffffff803d76d4>] tty_ioctl+0xbf4/0xfe0
May 11 10:59:48 [kernel] [ 2494.165880]        [<ffffffff802a05e1>] vfs_ioctl+0x31/0x90
May 11 10:59:48 [kernel] [ 2494.165888]        [<ffffffff802a06b3>] do_vfs_ioctl+0x73/0x2d0
May 11 10:59:48 [kernel] [ 2494.165895]        [<ffffffff802a095a>] sys_ioctl+0x4a/0x80
May 11 10:59:48 [kernel] [ 2494.165902]        [<ffffffff8020b5ab>] system_call_after_swapgs+0x7b/0x80
May 11 10:59:48 [kernel] [ 2494.165910]        [<ffffffffffffffff>] 0xffffffffffffffff
May 11 10:59:48 [kernel] [ 2494.165924]
May 11 10:59:48 [kernel] [ 2494.165925] -> #1 (&sighand->siglock){++..}:
May 11 10:59:48 [kernel] [ 2494.165929]        [<ffffffff80253796>] __lock_acquire+0xf86/0x1080
May 11 10:59:48 [kernel] [ 2494.165936]        [<ffffffff80253922>] lock_acquire+0x92/0xc0
May 11 10:59:48 [kernel] [ 2494.165943]        [<ffffffff804dec1f>] _spin_lock+0x2f/0x40
May 11 10:59:48 [kernel] [ 2494.165951]        [<ffffffff8022d5a3>] copy_process+0x973/0x1210
May 11 10:59:48 [kernel] [ 2494.165959]        [<ffffffff8022df12>] do_fork+0x82/0x2f0
May 11 10:59:48 [kernel] [ 2494.165967]        [<ffffffff8020bfe1>] kernel_thread+0x81/0xde
May 11 10:59:48 [kernel] [ 2494.165974]        [<ffffffff8020c048>] child_rip+0xa/0x12
May 11 10:59:48 [kernel] [ 2494.165981]        [<ffffffffffffffff>] 0xffffffffffffffff
May 11 10:59:48 [kernel] [ 2494.166038]
May 11 10:59:48 [kernel] [ 2494.166039] -> #0 (tasklist_lock){..??}:
May 11 10:59:48 [kernel] [ 2494.166043]        [<ffffffff802535ab>] __lock_acquire+0xd9b/0x1080
May 11 10:59:48 [kernel] [ 2494.166050]        [<ffffffff80253922>] lock_acquire+0x92/0xc0
May 11 10:59:48 [kernel] [ 2494.166057]        [<ffffffff804dede2>] _read_lock+0x32/0x50
May 11 10:59:48 [kernel] [ 2494.166063]        [<ffffffff80232e95>] is_current_pgrp_orphaned+0x15/0x50
May 11 10:59:48 [kernel] [ 2494.166071]        [<ffffffff803d5f80>] tty_check_change+0xb0/0x110
May 11 10:59:48 [kernel] [ 2494.166078]        [<ffffffff803dac5f>] set_termios+0x1f/0x4c0
May 11 10:59:48 [kernel] [ 2494.166085]        [<ffffffff803db379>] tty_mode_ioctl+0x279/0x3e0
May 11 10:59:48 [kernel] [ 2494.166092]        [<ffffffff803db51d>] n_tty_ioctl+0x3d/0x260
May 11 10:59:48 [kernel] [ 2494.166100]        [<ffffffff803d6c34>] tty_ioctl+0x154/0xfe0
May 11 10:59:48 [kernel] [ 2494.166107]        [<ffffffff802a05e1>] vfs_ioctl+0x31/0x90
May 11 10:59:48 [kernel] [ 2494.166114]        [<ffffffff802a06b3>] do_vfs_ioctl+0x73/0x2d0
May 11 10:59:48 [kernel] [ 2494.166121]        [<ffffffff802a095a>] sys_ioctl+0x4a/0x80
May 11 10:59:48 [kernel] [ 2494.166128]        [<ffffffff8020b5ab>] system_call_after_swapgs+0x7b/0x80
May 11 10:59:48 [kernel] [ 2494.166135]        [<ffffffffffffffff>] 0xffffffffffffffff
May 11 10:59:48 [kernel] [ 2494.166142]
May 11 10:59:48 [kernel] [ 2494.166143] other info that might help us debug this:
May 11 10:59:48 [kernel] [ 2494.166144]
May 11 10:59:48 [kernel] [ 2494.166146] 1 lock held by less/7053:
May 11 10:59:48 [kernel] [ 2494.166148]  #0:  (&tty->ctrl_lock){....}, at: [<ffffffff803d5f31>] tty_check_change+0x61/0x110
May 11 10:59:48 [kernel] [ 2494.166155]
May 11 10:59:48 [kernel] [ 2494.166156] stack backtrace:
May 11 10:59:48 [kernel] [ 2494.166159] Pid: 7053, comm: less Not tainted 2.6.26-rc1-00007-g91b3a7a #217
May 11 10:59:48 [kernel] [ 2494.166161]
May 11 10:59:48 [kernel] [ 2494.166162] Call Trace:
May 11 10:59:48 [kernel] [ 2494.166168]  [<ffffffff80251223>] print_circular_bug_tail+0x83/0x90
May 11 10:59:48 [kernel] [ 2494.166172]  [<ffffffff80250889>] ? print_circular_bug_entry+0x49/0x60
May 11 10:59:48 [kernel] [ 2494.166178]  [<ffffffff802535ab>] __lock_acquire+0xd9b/0x1080
May 11 10:59:48 [kernel] [ 2494.166184]  [<ffffffff80232e95>] ? is_current_pgrp_orphaned+0x15/0x50
May 11 10:59:48 [kernel] [ 2494.166189]  [<ffffffff80253922>] lock_acquire+0x92/0xc0
May 11 10:59:48 [kernel] [ 2494.166206]  [<ffffffff803d5f80>] tty_check_change+0xb0/0x110
May 11 10:59:48 [kernel] [ 2494.166211]  [<ffffffff803dac5f>] set_termios+0x1f/0x4c0
May 11 10:59:48 [kernel] [ 2494.166216]  [<ffffffff803d3423>] ? tty_ldisc_try+0x23/0x60
May 11 10:59:48 [kernel] [ 2494.166220]  [<ffffffff803d3444>] ? tty_ldisc_try+0x44/0x60
May 11 10:59:48 [kernel] [ 2494.166224]  [<ffffffff804df2c5>] ? _spin_unlock_irqrestore+0x65/0x80
May 11 10:59:48 [kernel] [ 2494.166230]  [<ffffffff803db379>] tty_mode_ioctl+0x279/0x3e0
May 11 10:59:48 [kernel] [ 2494.166234]  [<ffffffff803d3444>] ? tty_ldisc_try+0x44/0x60
May 11 10:59:48 [kernel] [ 2494.166239]  [<ffffffff803db51d>] n_tty_ioctl+0x3d/0x260
May 11 10:59:48 [kernel] [ 2494.166244]  [<ffffffff803d6c34>] tty_ioctl+0x154/0xfe0
May 11 10:59:48 [kernel] [ 2494.166249]  [<ffffffff80252baa>] ? __lock_acquire+0x39a/0x1080
May 11 10:59:48 [kernel] [ 2494.166256]  [<ffffffff80252baa>] ? __lock_acquire+0x39a/0x1080
May 11 10:59:48 [kernel] [ 2494.166263]  [<ffffffff80252baa>] ? __lock_acquire+0x39a/0x1080
May 11 10:59:48 [kernel] [ 2494.166269]  [<ffffffff802a05e1>] vfs_ioctl+0x31/0x90
May 11 10:59:48 [kernel] [ 2494.166274]  [<ffffffff802a06b3>] do_vfs_ioctl+0x73/0x2d0
May 11 10:59:48 [kernel] [ 2494.166280]  [<ffffffff802a095a>] sys_ioctl+0x4a/0x80
May 11 10:59:48 [kernel] [ 2494.166286]  [<ffffffff8020b5ab>] system_call_after_swapgs+0x7b/0x80
May 11 10:59:48 [kernel] [ 2494.166292]
Acked-by: NAlan Cox <alan@lxorguk.ukuu.org.uk>
Reported-by: NMarcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/tty_io.c

@@ -1215,10 +1215,11 @@ int tty_check_change(struct tty_struct *tty)
 
 	if (!tty->pgrp) {
 		printk(KERN_WARNING "tty_check_change: tty->pgrp == NULL!\n");
-		goto out;
+		goto out_unlock;
 	}
 	if (task_pgrp(current) == tty->pgrp)
-		goto out;
+		goto out_unlock;
+	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 	if (is_ignored(SIGTTOU))
 		goto out;
 	if (is_current_pgrp_orphaned()) {
@@ -1229,6 +1230,8 @@ int tty_check_change(struct tty_struct *tty)
 	set_thread_flag(TIF_SIGPENDING);
 	ret = -ERESTARTSYS;
 out:
+	return ret;
+out_unlock:
 	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 	return ret;
 }