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Date:   Thu, 25 Jul 2019 13:26:51 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Martin =?iso-8859-1?Q?Hundeb=F8ll?= <martin@geanix.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Jiri Slaby <jslaby@suse.com>,
        Dirkjan Bussink <dirkjan.bussink@nedap.com>
Subject: Re: [BUG] n_gsm: possible recursive locking detected
Message-ID: <20190725112651.GA12309@kroah.com>
References: <e2e9cc8f-6bb4-f66f-2d2b-3875c2e66cd3@geanix.com>
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On Wed, Jul 17, 2019 at 11:40:02AM +0200, Martin Hundeb?ll 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?

thanks,

greg k-h