[SRU][Trusty][V2][PATCH 1/1] tty: fix stall caused by missing memory barrier in drivers/tty/n_tty.c

[Stefan Bader]

On 18.11.2015 17:25, Joseph Salisbury wrote:
> From: Kosuke Tatsukawa <tatsu at ab.jp.nec.com>
> 
> BugLink: http://bugs.launchpad.net/bugs/1512815
> 
> My colleague ran into a program stall on a x86_64 server, where
> n_tty_read() was waiting for data even if there was data in the buffer
> in the pty.  kernel stack for the stuck process looks like below.
>  #0 [ffff88303d107b58] __schedule at ffffffff815c4b20
>  #1 [ffff88303d107bd0] schedule at ffffffff815c513e
>  #2 [ffff88303d107bf0] schedule_timeout at ffffffff815c7818
>  #3 [ffff88303d107ca0] wait_woken at ffffffff81096bd2
>  #4 [ffff88303d107ce0] n_tty_read at ffffffff8136fa23
>  #5 [ffff88303d107dd0] tty_read at ffffffff81368013
>  #6 [ffff88303d107e20] __vfs_read at ffffffff811a3704
>  #7 [ffff88303d107ec0] vfs_read at ffffffff811a3a57
>  #8 [ffff88303d107f00] sys_read at ffffffff811a4306
>  #9 [ffff88303d107f50] entry_SYSCALL_64_fastpath at ffffffff815c86d7
> 
> There seems to be two problems causing this issue.
> 
> First, in drivers/tty/n_tty.c, __receive_buf() stores the data and
> updates ldata->commit_head using smp_store_release() and then checks
> the wait queue using waitqueue_active().  However, since there is no
> memory barrier, __receive_buf() could return without calling
> wake_up_interactive_poll(), and at the same time, n_tty_read() could
> start to wait in wait_woken() as in the following chart.
> 
>         __receive_buf()                         n_tty_read()
> ------------------------------------------------------------------------
> if (waitqueue_active(&tty->read_wait))
> /* Memory operations issued after the
>    RELEASE may be completed before the
>    RELEASE operation has completed */
>                                         add_wait_queue(&tty->read_wait, &wait);
>                                         ...
>                                         if (!input_available_p(tty, 0)) {
> smp_store_release(&ldata->commit_head,
>                   ldata->read_head);
>                                         ...
>                                         timeout = wait_woken(&wait,
>                                           TASK_INTERRUPTIBLE, timeout);
> ------------------------------------------------------------------------
> 
> The second problem is that n_tty_read() also lacks a memory barrier
> call and could also cause __receive_buf() to return without calling
> wake_up_interactive_poll(), and n_tty_read() to wait in wait_woken()
> as in the chart below.
> 
>         __receive_buf()                         n_tty_read()
> ------------------------------------------------------------------------
>                                         spin_lock_irqsave(&q->lock, flags);
>                                         /* from add_wait_queue() */
>                                         ...
>                                         if (!input_available_p(tty, 0)) {
>                                         /* Memory operations issued after the
>                                            RELEASE may be completed before the
>                                            RELEASE operation has completed */
> smp_store_release(&ldata->commit_head,
>                   ldata->read_head);
> if (waitqueue_active(&tty->read_wait))
>                                         __add_wait_queue(q, wait);
>                                         spin_unlock_irqrestore(&q->lock,flags);
>                                         /* from add_wait_queue() */
>                                         ...
>                                         timeout = wait_woken(&wait,
>                                           TASK_INTERRUPTIBLE, timeout);
> ------------------------------------------------------------------------
> 
> There are also other places in drivers/tty/n_tty.c which have similar
> calls to waitqueue_active(), so instead of adding many memory barrier
> calls, this patch simply removes the call to waitqueue_active(),
> leaving just wake_up*() behind.
> 
> This fixes both problems because, even though the memory access before
> or after the spinlocks in both wake_up*() and add_wait_queue() can
> sneak into the critical section, it cannot go past it and the critical
> section assures that they will be serialized (please see "INTER-CPU
> ACQUIRING BARRIER EFFECTS" in Documentation/memory-barriers.txt for a
> better explanation).  Moreover, the resulting code is much simpler.
> 
> Latency measurement using a ping-pong test over a pty doesn't show any
> visible performance drop.
> 
> Signed-off-by: Kosuke Tatsukawa <tatsu at ab.jp.nec.com>
> Cc: stable at vger.kernel.org
> Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> (backported from commit e81107d4c6bd098878af9796b24edc8d4a9524fd)
> Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
> ---
>  drivers/tty/n_tty.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index 84dcdf4..3de41df 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -1385,8 +1385,7 @@ handle_newline:
>  			put_tty_queue(c, ldata);
>  			ldata->canon_head = ldata->read_head;
>  			kill_fasync(&tty->fasync, SIGIO, POLL_IN);
> -			if (waitqueue_active(&tty->read_wait))
> -				wake_up_interruptible(&tty->read_wait);
> +			wake_up_interruptible_poll(&tty->read_wait, POLLIN);

Are you sure about changing the wake up into a wakeup_*_poll? The original patch
patch only removed if cases but left the type of wakup as it was before...

>  			return 0;
>  		}
>  	}
> @@ -1671,8 +1670,7 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
>  	if ((!ldata->icanon && (read_cnt(ldata) >= ldata->minimum_to_wake)) ||
>  		L_EXTPROC(tty)) {
>  		kill_fasync(&tty->fasync, SIGIO, POLL_IN);
> -		if (waitqueue_active(&tty->read_wait))
> -			wake_up_interruptible(&tty->read_wait);
> +		wake_up_interruptible_poll(&tty->read_wait, POLLIN);

Same here...

-Stefan

>  	}
>  }
>  
> 


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