Re: [BUG] n_gsm: possible recursive locking detected

[Martin HundebÃll]

On 25/07/2019 13.26, Greg Kroah-Hartman wrote:
> On Wed, Jul 17, 2019 at 11:40:02AM +0200, Martin HundebÃll wrote:
> > Hi,
> >
> > The GSM0710 line discipline driver triggers a lockdep warning when disabling
> > the ldisc while holding a multiplexed virtual tty open:
> >
> > ============================================
> > WARNING: possible recursive locking detected
> > 5.2.0-00114-gdab52e30156b #6 Not tainted
> > --------------------------------------------
> > cmux/263 is trying to acquire lock:
> > e1e23b18 (&tty->legacy_mutex){+.+.}, at: __tty_hangup.part.0+0x58/0x27c
> >
> > but task is already holding lock:
> > d6eddf48 (&tty->legacy_mutex){+.+.}, at: tty_set_ldisc+0x3c/0x1bc
> >
> > other info that might help us debug this:
> >   Possible unsafe locking scenario:
> >
> >         CPU0
> >         ----
> >    lock(&tty->legacy_mutex);
> >    lock(&tty->legacy_mutex);
> >
> >   *** DEADLOCK ***
> >
> >   May be due to missing lock nesting notation
> >
> > 3 locks held by cmux/263:
> >   #0: d6eddf48 (&tty->legacy_mutex){+.+.}, at: tty_set_ldisc+0x3c/0x1bc
> >   #1: f28bead9 (&tty->ldisc_sem){++++}, at: tty_ldisc_lock+0x50/0x74
> >   #2: e5d20e4f (&gsm->mutex){+.+.}, at: gsm_cleanup_mux+0x9c/0x15c
> >
> > stack backtrace:
> > CPU: 0 PID: 263 Comm: cmux Not tainted 5.2.0-00114-gdab52e30156b #6
> > Hardware name: Freescale i.MX6 Ultralite (Device Tree)
> > [<c011184c>] (unwind_backtrace) from [<c010cc74>] (show_stack+0x10/0x14)
> > [<c010cc74>] (show_stack) from [<c0852488>] (dump_stack+0xd4/0x108)
> > [<c0852488>] (dump_stack) from [<c017be90>] (__lock_acquire+0x6ec/0x1e84)
> > [<c017be90>] (__lock_acquire) from [<c017ddc4>] (lock_acquire+0xcc/0x204)
> > [<c017ddc4>] (lock_acquire) from [<c086e9d0>] (__mutex_lock+0x64/0x90c)
> > [<c086e9d0>] (__mutex_lock) from [<c086f294>] (mutex_lock_nested+0x1c/0x24)
> > [<c086f294>] (mutex_lock_nested) from [<c04c02fc>]
> > (__tty_hangup.part.0+0x58/0x27c)
> > [<c04c02fc>] (__tty_hangup.part.0) from [<c04ce718>]
> > (gsm_cleanup_mux+0xe8/0x15c)
> > [<c04ce718>] (gsm_cleanup_mux) from [<c04ce7d4>] (gsmld_close+0x48/0x90)
> > [<c04ce7d4>] (gsmld_close) from [<c04c7e24>] (tty_set_ldisc+0xb8/0x1bc)
> > [<c04c7e24>] (tty_set_ldisc) from [<c04c0b70>] (tty_ioctl+0x640/0xcb0)
> > [<c04c0b70>] (tty_ioctl) from [<c0297e68>] (do_vfs_ioctl+0x41c/0xa5c)
> > [<c0297e68>] (do_vfs_ioctl) from [<c02984dc>] (ksys_ioctl+0x34/0x60)
> > [<c02984dc>] (ksys_ioctl) from [<c0101000>] (ret_fast_syscall+0x0/0x28)
> > Exception stack(0xc8ce1fa8 to 0xc8ce1ff0)
> > 1fa0:                   00438000 00000000 00000003 00005423 beb6cc04
> > beb6cc04
> > 1fc0: 00438000 00000000 00000000 00000036 00000000 00000000 00438000
> > beb6ccd4
> > 1fe0: 00438048 beb6cbfc 00427684 b6f58b88
> >
> >
> > Steps to reproduce using the attached cmux util:
> >
> > root@iwg26:~# ./cmux &
> > [1] 254
> > SERIAL_PORT = /dev/ttymxc0
> > AT+IFC=2: Ie5   +CFUN: 1    +CPIN: READY    Call Ready  AT+IFC=2,2   OK
> > AT+GMM  : AT+GMM   Quectel_M95    OK
> > AT      : AT   OK
> > AT+IPR=1: AT+IPR=115200&w   OK
> > AT+CMUX=: AT+CMUX=0,0,5,512,10,3,30,10,2   OK
> > Line dicipline set
> >
> > root@iwg26:~# cat /dev/gsmtty1 &
> > [2] 262
> > root@iwg26:~# kill %1
> > [   74.517449] ============================================
> > [   74.522769] WARNING: possible recursive locking detected
> > [   74.528094] 5.2.0-00114-gdab52e30156b #6 Not tainted
> > [   74.533065] --------------------------------------------
> > <...>
> >
> >
> > This has supposedly been fixed before in 4d9b109060f6 ("tty: Prevent
> > deadlock in n_gsm driver"), but the fix was undone in be7065725590
> > ("TTY/n_gsm: Removing the wrong tty_unlock/lock() in gsm_dlci_release()")
>
> Do you have a patch that can resolve this given you have a test case?

No, sorry.

I can try to cook a patch, but chances are I will break locking for 
someone else. Hints are welcome.

// Martin