Date:   Thu, 23 Mar 2017 14:22:08 -0700
From:   lkml@...garu.com
To:     Rob Herring <robh@...nel.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] 4.11.0-rc3 xterm hung in D state on exit, wchan is
 tty_release_struct

On Thu, Mar 23, 2017 at 11:57:22AM -0500, Rob Herring wrote:
> On Thu, Mar 23, 2017 at 08:46:03AM -0500, Rob Herring wrote:
> > On Thu, Mar 23, 2017 at 12:30:18AM -0700, lkml@...garu.com wrote:
> > > On Wed, Mar 22, 2017 at 11:44:18PM -0700, lkml@...garu.com wrote:
> > > > On Wed, Mar 22, 2017 at 07:08:46PM -0700, lkml@...garu.com wrote:
> > > > > Hello list,
> > > > > 
> > > > > After approximately one day day of running 4.11.0-rc3 with 7e54d9d reverted to
> > > > > enable regular use, this happened upon destroying an xterm:
> > > > > 
> 
> [...]
> 
> > > > 
> > > > Added Rob Herring, author of c3485ee to CC list.
> > > > 
> > > 
> > > I suspect this part was a mistake:
> > > 
> > >  -       tty = READ_ONCE(port->itty);
> > >  -       if (tty == NULL)
> > >  -               return;
> > > 
> > > Note release_tty() tty->port->itty is assigned NULL before calling
> > > tty_buffer_cancel_work():
> > 
> > The READ_ONCE should still handle that.
> > 
> > Anyway, the changes were purely to try to remove the need for a ldisc in 
> > the serdev case and avoid referencing it. In fact we still have an 
> > ldisc, it's just not used. So we can restore the original ordering.
> > 
> > Can you try this patch:
> > 
> 
> Please try this one instead. It passes the tty struct around instead of 
> the ldisc.
> 

Happy to test the fix, except reproducing the bug without changing anything
at all has proven elusive.  AFAIK we just have my single experience
described in the report, I'm unconfident in my ability to validate any
fixes for this specific bug.

If you think you understand the root cause, maybe you can conceive of a
more reliable reproducer than me just using my machine?  It appears I may
have just been very (un)lucky.

Thanks,
Vito Caputo


> 8<---------------------------------------------------------------------
> >From 4d727a92267541aeba172ee52d9b771d7176297c Mon Sep 17 00:00:00 2001
> From: Rob Herring <robh@...nel.org>
> Date: Thu, 23 Mar 2017 08:50:10 -0500
> Subject: [PATCH] tty: fix regression in flush_to_ldisc
> 
> Commit c3485ee0d560 ("tty_port: Add port client functions") rearranged
> getting the reference to the ldisc. Vito Caputo reports:
> 
> "This happened upon destroying an xterm:
> 
> [80817.525112] BUG: unable to handle kernel paging request at 0000000000002260
> [80817.525239] IP: n_tty_receive_buf_common+0x68/0xab0
> [80817.525312] PGD 0
> 
> [80817.525387] Oops: 0000 [#1] PREEMPT SMP
> [80817.525452] CPU: 0 PID: 9532 Comm: kworker/u4:3 Not tainted 4.11.0-rc3-00001-gc56a355 #53
> [80817.525564] Hardware name: LENOVO 7668CTO/7668CTO, BIOS 7NETC2WW (2.22 ) 03/22/2011
> [80817.525673] Workqueue: events_unbound flush_to_ldisc
> [80817.525752] task: ffff967d91d80000 task.stack: ffff9add81f40000
> [80817.525839] RIP: 0010:n_tty_receive_buf_common+0x68/0xab0
> [80817.525917] RSP: 0018:ffff9add81f43d38 EFLAGS: 00010297
> [80817.525992] RAX: 0000000000000000 RBX: ffff967d91c98c00 RCX: 0000000000000001
> [80817.526035] RDX: ffff967e73bba58d RSI: ffff967e73bba48d RDI: ffff967d91c98cc0
> [80817.526035] RBP: ffff9add81f43dd0 R08: 0000000000000001 R09: 0000000000000000
> [80817.526035] R10: 00004980cbe001e0 R11: 0000000000000000 R12: ffff967d87aacf20
> [80817.526035] R13: ffff967e73bba58d R14: 0000000000000001 R15: ffff967e74aa8008
> [80817.526035] FS:  0000000000000000(0000) GS:ffff967e7bc00000(0000) knlGS:0000000000000000
> [80817.526035] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [80817.526035] CR2: 0000000000002260 CR3: 0000000099009000 CR4: 00000000000006f0
> [80817.526035] Call Trace:
> [80817.526035]  ? update_curr+0xbb/0x1a0
> [80817.526035]  n_tty_receive_buf2+0xf/0x20
> [80817.526035]  tty_ldisc_receive_buf+0x1d/0x50
> [80817.526035]  tty_port_default_receive_buf+0x40/0x60
> [80817.526035]  flush_to_ldisc+0x94/0xa0
> [80817.526035]  process_one_work+0x13b/0x3e0
> [80817.526035]  worker_thread+0x64/0x4a0
> [80817.526035]  kthread+0x10f/0x150
> [80817.526035]  ? process_one_work+0x3e0/0x3e0
> [80817.526035]  ? __kthread_create_on_node+0x150/0x150
> [80817.526035]  ret_from_fork+0x29/0x40
> [80817.526035] Code: 85 70 ff ff ff e8 59 75 57 00 48 8d 83 00 02 00 00 c7 45 c8 00 00 00 00 48 89 45 98 48 8d 83 00 02 00 00 48 89 45 90 48 8b 45 b8 <48> 8b b0 60 22 00 00 48 8b 08 89 f0 29 c8 f6 83 10 01 00 00 08
> [80817.526035] RIP: n_tty_receive_buf_common+0x68/0xab0 RSP: ffff9add81f43d38
> [80817.526035] CR2: 0000000000002260
> [80817.526035] ---[ end trace 640aec4765d350f2 ]---
> 
> That xterm process is stuck, and I am unable to start any new xterms,
> switching to virtual consoles proves useless, presumably there's an
> important lock held."
> 
> Revert the changes in flush_to_ldisc so that we take a ref to the ldisc
> in the beginning. This requires the tty struct to be passed to the
> receive_buf client functions instead of ideally using the tty_port as we
> can't reliably retrieve the struct tty from the tty_port. However, since
> we do hold a reference to the ldisc, we can safely access tty->ldisc
> more than once.
> 
> Reported-by: Vito Caputo <lkml@...garu.com>
> Fixes: c3485ee0d560 ("tty_port: Add port client functions")
> Signed-off-by: Rob Herring <robh@...nel.org>
> ---
>  drivers/tty/serdev/serdev-ttyport.c |  4 ++--
>  drivers/tty/tty_buffer.c            | 17 ++++++++++++++---
>  drivers/tty/tty_port.c              | 20 ++------------------
>  include/linux/tty.h                 |  2 +-
>  4 files changed, 19 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/tty/serdev/serdev-ttyport.c b/drivers/tty/serdev/serdev-ttyport.c
> index d05393594f15..93872057335f 100644
> --- a/drivers/tty/serdev/serdev-ttyport.c
> +++ b/drivers/tty/serdev/serdev-ttyport.c
> @@ -29,10 +29,10 @@ struct serport {
>   * Callback functions from the tty port.
>   */
>  
> -static int ttyport_receive_buf(struct tty_port *port, const unsigned char *cp,
> +static int ttyport_receive_buf(struct tty_struct *tty, const unsigned char *cp,
>  				const unsigned char *fp, size_t count)
>  {
> -	struct serdev_controller *ctrl = port->client_data;
> +	struct serdev_controller *ctrl = tty->port->client_data;
>  	struct serport *serport = serdev_controller_get_drvdata(ctrl);
>  
>  	if (!test_bit(SERPORT_ACTIVE, &serport->flags))
> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> index 4e7a4e9dcf4d..71a65f98aca8 100644
> --- a/drivers/tty/tty_buffer.c
> +++ b/drivers/tty/tty_buffer.c
> @@ -437,7 +437,7 @@ int tty_ldisc_receive_buf(struct tty_ldisc *ld, const unsigned char *p,
>  EXPORT_SYMBOL_GPL(tty_ldisc_receive_buf);
>  
>  static int
> -receive_buf(struct tty_port *port, struct tty_buffer *head, int count)
> +receive_buf(struct tty_struct *tty, struct tty_buffer *head, int count)
>  {
>  	unsigned char *p = char_buf_ptr(head, head->read);
>  	char	      *f = NULL;
> @@ -445,7 +445,7 @@ receive_buf(struct tty_port *port, struct tty_buffer *head, int count)
>  	if (~head->flags & TTYB_NORMAL)
>  		f = flag_buf_ptr(head, head->read);
>  
> -	return port->client_ops->receive_buf(port, p, f, count);
> +	return tty->port->client_ops->receive_buf(tty, p, f, count);
>  }
>  
>  /**
> @@ -465,6 +465,16 @@ static void flush_to_ldisc(struct work_struct *work)
>  {
>  	struct tty_port *port = container_of(work, struct tty_port, buf.work);
>  	struct tty_bufhead *buf = &port->buf;
> +	struct tty_struct *tty;
> +	struct tty_ldisc *disc;
> +
> +	tty = READ_ONCE(port->itty);
> +	if (tty == NULL)
> +		return;
> +
> +	disc = tty_ldisc_ref(tty);
> +	if (disc == NULL)
> +		return;
>  
>  	mutex_lock(&buf->lock);
>  
> @@ -494,7 +504,7 @@ static void flush_to_ldisc(struct work_struct *work)
>  			continue;
>  		}
>  
> -		count = receive_buf(port, head, count);
> +		count = receive_buf(tty, head, count);
>  		if (!count)
>  			break;
>  		head->read += count;
> @@ -502,6 +512,7 @@ static void flush_to_ldisc(struct work_struct *work)
>  
>  	mutex_unlock(&buf->lock);
>  
> +	tty_ldisc_deref(disc);
>  }
>  
>  /**
> diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
> index 1d21a9c1d33e..44178684d40d 100644
> --- a/drivers/tty/tty_port.c
> +++ b/drivers/tty/tty_port.c
> @@ -18,27 +18,11 @@
>  #include <linux/module.h>
>  #include <linux/serdev.h>
>  
> -static int tty_port_default_receive_buf(struct tty_port *port,
> +static int tty_port_default_receive_buf(struct tty_struct *tty,
>  					const unsigned char *p,
>  					const unsigned char *f, size_t count)
>  {
> -	int ret;
> -	struct tty_struct *tty;
> -	struct tty_ldisc *disc;
> -
> -	tty = READ_ONCE(port->itty);
> -	if (!tty)
> -		return 0;
> -
> -	disc = tty_ldisc_ref(tty);
> -	if (!disc)
> -		return 0;
> -
> -	ret = tty_ldisc_receive_buf(disc, p, (char *)f, count);
> -
> -	tty_ldisc_deref(disc);
> -
> -	return ret;
> +	return tty_ldisc_receive_buf(tty->ldisc, p, (char *)f, count);
>  }
>  
>  static void tty_port_default_wakeup(struct tty_port *port)
> diff --git a/include/linux/tty.h b/include/linux/tty.h
> index 1017e904c0a3..e1400b0562b6 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -219,7 +219,7 @@ struct tty_port_operations {
>  };
>  
>  struct tty_port_client_operations {
> -	int (*receive_buf)(struct tty_port *port, const unsigned char *, const unsigned char *, size_t);
> +	int (*receive_buf)(struct tty_struct *tty, const unsigned char *, const unsigned char *, size_t);
>  	void (*write_wakeup)(struct tty_port *port);
>  };
>  
> -- 
> 2.10.1

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