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Subject: Re: [PATCH] tty: n_gsm: avoid recursive locking with async port
 hangup
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        linux-kernel@vger.kernel.org
Cc:     =?UTF-8?Q?Sean_Nyekj=c3=a6r?= <sean@geanix.com>,
        Esben Haabendal <esben@geanix.com>
References: <20190822215601.9028-1-martin@geanix.com>
From:   =?UTF-8?Q?Martin_Hundeb=c3=b8ll?= <martin@geanix.com>
Message-ID: <4fd2d737-14a8-6fe6-16a1-c5e6d924f9e6@geanix.com>
Date:   Thu, 29 Aug 2019 21:42:31 +0200
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On 22/08/2019 23.56, Martin Hundebøll wrote:
> When tearing down the n_gsm ldisc while one or more of its child ports
> are open, a lock dep warning occurs:
> 
> [   56.254258] ======================================================
> [   56.260447] WARNING: possible circular locking dependency detected
> [   56.266641] 5.2.0-00118-g1fd58e20e5b0 #30 Not tainted
> [   56.271701] ------------------------------------------------------
> [   56.277890] cmux/271 is trying to acquire lock:
> [   56.282436] 8215283a (&tty->legacy_mutex){+.+.}, at: __tty_hangup.part.0+0x58/0x27c
> [   56.290128]
> [   56.290128] but task is already holding lock:
> [   56.295970] e9e2b842 (&gsm->mutex){+.+.}, at: gsm_cleanup_mux+0x9c/0x15c
> [   56.302699]
> [   56.302699] which lock already depends on the new lock.
> [   56.302699]
> [   56.310884]
> [   56.310884] the existing dependency chain (in reverse order) is:
> [   56.318372]
> [   56.318372] -> #2 (&gsm->mutex){+.+.}:
> [   56.323624]        mutex_lock_nested+0x1c/0x24
> [   56.328079]        gsm_cleanup_mux+0x9c/0x15c
> [   56.332448]        gsmld_ioctl+0x418/0x4e8
> [   56.336554]        tty_ioctl+0x96c/0xcb0
> [   56.340492]        do_vfs_ioctl+0x41c/0xa5c
> [   56.344685]        ksys_ioctl+0x34/0x60
> [   56.348535]        ret_fast_syscall+0x0/0x28
> [   56.352815]        0xbe97cc04
> [   56.355791]
> [   56.355791] -> #1 (&tty->ldisc_sem){++++}:
> [   56.361388]        tty_ldisc_lock+0x50/0x74
> [   56.365581]        tty_init_dev+0x88/0x1c4
> [   56.369687]        tty_open+0x1c8/0x430
> [   56.373536]        chrdev_open+0xa8/0x19c
> [   56.377560]        do_dentry_open+0x118/0x3c4
> [   56.381928]        path_openat+0x2fc/0x1190
> [   56.386123]        do_filp_open+0x68/0xd4
> [   56.390146]        do_sys_open+0x164/0x220
> [   56.394257]        kernel_init_freeable+0x328/0x3e4
> [   56.399146]        kernel_init+0x8/0x110
> [   56.403078]        ret_from_fork+0x14/0x20
> [   56.407183]        0x0
> [   56.409548]
> [   56.409548] -> #0 (&tty->legacy_mutex){+.+.}:
> [   56.415402]        __mutex_lock+0x64/0x90c
> [   56.419508]        mutex_lock_nested+0x1c/0x24
> [   56.423961]        __tty_hangup.part.0+0x58/0x27c
> [   56.428676]        gsm_cleanup_mux+0xe8/0x15c
> [   56.433043]        gsmld_close+0x48/0x90
> [   56.436979]        tty_ldisc_kill+0x2c/0x6c
> [   56.441173]        tty_ldisc_release+0x88/0x194
> [   56.445715]        tty_release_struct+0x14/0x44
> [   56.450254]        tty_release+0x36c/0x43c
> [   56.454365]        __fput+0x94/0x1e8
> 
> Avoid the warning by doing the port hangup asynchronously.

Any comments?

// Martin

> Signed-off-by: Martin Hundebøll <martin@geanix.com>
> ---
>   drivers/tty/n_gsm.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index d30525946892..36a3eb4ad4c5 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -1716,7 +1716,7 @@ static void gsm_dlci_release(struct gsm_dlci *dlci)
>   		gsm_destroy_network(dlci);
>   		mutex_unlock(&dlci->mutex);
>   
> -		tty_vhangup(tty);
> +		tty_hangup(tty);
>   
>   		tty_port_tty_set(&dlci->port, NULL);
>   		tty_kref_put(tty);
>