From 000b9151d7851cc1e490b2a76d0206e524f43cca Mon Sep 17 00:00:00 2001 From: Christian Borntraeger Date: Mon, 11 Aug 2008 09:02:49 +0100 Subject: [PATCH] Fix race/oops in tty layer after BKL pushdown While testing our KVM code for s390 (starting and killall kvm in a loop) I can reproduce the following oops: Unable to handle kernel pointer dereference at virtual kernel address 6b6b6b6b6b6b6000 Oops: 0038 [#1] SMP Modules linked in: dm_multipath sunrpc qeth_l3 qeth_l2 dm_mod qeth ccwgroup CPU: 1 Not tainted 2.6.27-rc1 #54 Process kuli (pid: 4409, task: 00000000b6aa5940, ksp: 00000000b7343e10) Krnl PSW : 0704e00180000000 00000000002e0b8c (disassociate_ctty+0x1c0/0x288) R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 EA:3 Krnl GPRS: 0000000000000000 6b6b6b6b6b6b6b6b 0000000000000001 00000000000003a6 00000000002e0a46 00000000004b4160 0000000000000001 00000000bbd79758 00000000b7343e58 00000000b8854148 00000000bd34dea0 00000000b7343c20 0000000000000001 00000000004b6d08 00000000002e0a46 00000000b7343c20 Krnl Code: 00000000002e0b7e: eb9fb0a00004 lmg %r9,%r15,160(%r11) 00000000002e0b84: 07f4 bcr 15,%r4 00000000002e0b86: e31090080004 lg %r1,8(%r9) >00000000002e0b8c: d501109cd000 clc 156(2,%r1),0(%r13) 00000000002e0b92: a784ff5d brc 8,2e0a4c 00000000002e0b96: b9040029 lgr %r2,%r9 00000000002e0b9a: c0e5fffff9c3 brasl %r14,2dff20 00000000002e0ba0: a7f4ff56 brc 15,2e0a4c Call Trace: ([<00000000002e0a46>] disassociate_ctty+0x7a/0x288) [<0000000000141fe6>] do_exit+0x212/0x8d4 [<0000000000142708>] do_group_exit+0x60/0xcc [<0000000000150660>] get_signal_to_deliver+0x270/0x3ac [<000000000010bfd6>] do_signal+0x8e/0x8dc [<0000000000113772>] sysc_sigpending+0xe/0x22 [<000001ff0000b134>] 0x1ff0000b134 INFO: lockdep is turned off. Last Breaking-Event-Address: [<00000000002e0a48>] disassociate_ctty+0x7c/0x288 Kernel panic - not syncing: Fatal exception: panic_on_oops It seems that tty was already free in disassocate_ctty when it tries to dereference tty->driver. After moving the lock_kernel before the mutex_unlock, I can no longer reproduce the problem. [ This is a temporary partial fix for the documented and long standing race in disassociate_tty. This stops most problem cases for now. For the next release the -next tree has an initial implementation of kref counting for tty structures and this quickfix will be dropped. - Alan ] Signed-off-by: Christian Borntraeger Signed-off-by; Alan Cox Signed-off-by: Linus Torvalds --- drivers/char/tty_io.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c index e1b46bc..0e6866f 100644 --- a/drivers/char/tty_io.c +++ b/drivers/char/tty_io.c @@ -1161,8 +1161,8 @@ void disassociate_ctty(int on_exit) tty = get_current_tty(); if (tty) { tty_pgrp = get_pid(tty->pgrp); - mutex_unlock(&tty_mutex); lock_kernel(); + mutex_unlock(&tty_mutex); /* XXX: here we race, there is nothing protecting tty */ if (on_exit && tty->driver->type != TTY_DRIVER_TYPE_PTY) tty_vhangup(tty); -- 2.7.4