Date: Fri, 13 Jul 2007 09:35:04 +0200 From: Attilio Rao <attilio@FreeBSD.org> To: Julian Elischer <julian@elischer.org> Cc: FreeBSD Current <current@freebsd.org> Subject: Re: crash in tty code in 6.1.. fixed since? Message-ID: <46972B28.1010409@FreeBSD.org> In-Reply-To: <46970DF7.3000803@elischer.org> References: <46970DF7.3000803@elischer.org>
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Julian Elischer wrote:
> Does this crash look familiar to anyone?
> It's in 6.1 unfortunatly..
>
> #6 0xc069ca6a in calltrap () at ../../../i386/i386/exception.s:139
> #7 0xc0587012 in ttymodem (tp=0xc6990800, flag=-1065963704)
> at ../../../kern/tty.c:1659
> #8 0xc058b83e in ptcclose (dev=0x0, flags=7, fmt=8192, td=0xc6966d80)
> at linedisc.h:136
> #9 0xc052bf77 in giant_close (dev=0xc7547c00, fflag=7, devtype=8192,
> td=0xc6966d80) at ../../../kern/kern_conf.c:266
> #10 0xc051685f in devfs_close (ap=0xe706baa4)
> at ../../../fs/devfs/devfs_vnops.c:287
> #11 0xc06c2a10 in VOP_CLOSE_APV (vop=0x0, a=0xc076af48) at vnode_if.c:426
> #12 0xc05bf3ce in vn_close (vp=0xc764e550, flags=7, file_cred=0x0,
> td=0xc6966d80) at vnode_if.h:227
> #13 0xc05c0212 in vn_closefile (fp=0xc7532510, td=0xc6966d80)
> at ../../../kern/vfs_vnops.c:852
> #14 0xc0516887 in devfs_close_f (fp=0xc7532510, td=0xc6966d80)
> at ../../../fs/devfs/devfs_vnops.c:297
> #15 0xc05361e8 in fdrop_locked (fp=0xc7532510, td=0xc6966d80) at file.h:289
> #16 0xc0536135 in fdrop (fp=0xc7532510, td=0xc6966d80)
> at ../../../kern/kern_descrip.c:2122
> #17 0xc05346d3 in closef (fp=0xc7532510, td=0xc6966d80)
> at ../../../kern/kern_descrip.c:1942
> #18 0xc0533487 in fdfree (td=0xc6966d80) at
> ../../../kern/kern_descrip.c:1627
> #19 0xc053cc88 in exit1 (td=0xc6966d80, rv=15) at
> ../../../kern/kern_exit.c:263
> #20 0xc055b58b in sigexit (td=0xc6966d80, sig=15)
> at ../../../kern/kern_sig.c:2451
> #21 0xc055b296 in postsig (sig=15) at ../../../kern/kern_sig.c:2326
> #22 0xc0577fbe in ast (framep=0xe706bd38) at ../../../kern/subr_trap.c:266
> #23 0xc069d3ad in doreti_ast () at ../../../i386/i386/exception.s:293
>
> (kgdb) up
> #7 0xc0587012 in ttymodem (tp=0xc6990800, flag=-1065963704)
> at ../../../kern/tty.c:1659
> 1659 if (tp->t_session->s_leader) {
> Current language: auto; currently c
> (kgdb) list
> 1654 !ISSET(tp->t_cflag, CLOCAL)) {
> 1655 SET(tp->t_state, TS_ZOMBIE);
> 1656 CLR(tp->t_state, TS_CONNECTED);
> 1657 if (tp->t_session) {
> 1658 sx_slock(&proctree_lock);
> 1659 if (tp->t_session->s_leader) {
> 1660 struct proc *p;
> 1661
> 1662 p =
> tp->t_session->s_leader;
> 1663 PROC_LOCK(p);
>
> (kgdb) set print pretty
> (kgdb) p *tp
> $3 = {
> t_rawq = {
> c_cc = 0,
> [...]
>
> t_outcc = 119661, t_line = 0, t_dev = 0xc763fe00, t_mdev = 0x0,
> t_devunit = 0, t_state = 0, t_flags = 0, t_timeout = 300000, t_pgrp
> = 0x0, t_session = 0x0, t_sigio = 0x0, t_rsel = {
> si_thrlist = {
> tqe_next = 0x0, tqe_prev = 0x0
> }, si_thread = 0x0, si_note = {
> [...]
>
>
> tp_session is NULL but it shouldn't have been able to have run that line
> (line 1659) if it had tested NULL 2 lines before..
> this suggests a locking problem..
I think it has been fixed some months ago IIRC.
The problem here, should be that if sx_slock() let thread sleep, Giant
is released before to sleep and tp->t_session can be accessed in racy way.
Another nice side-effect about having tty Giant :)
Attilio
PS: I think I alredy documented these kinds of bugs in my locking
documentation in perforce.
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