linux-kernel - Re: linux-next: manual merge of the tty tree with the tty.current tree

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Date:   Mon, 20 Mar 2017 10:26:43 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Stephen Rothwell <sfr@...b.auug.org.au>
Cc:     Greg KH <greg@...ah.com>, linux-next@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Hurley <peter@...leysoftware.com>,
        Michael Neuling <mikey@...ling.org>
Subject: Re: linux-next: manual merge of the tty tree with the tty.current tree

On Mon, Mar 20, 2017 at 10:21 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Mon, Mar 20, 2017 at 3:28 AM, Stephen Rothwell <sfr@...b.auug.org.au> wrote:
>> Hi Greg,
>>
>> Today's linux-next merge of the tty tree got a conflict in:
>>
>>   drivers/tty/tty_ldisc.c
>>
>> between commit:
>>
>>   5362544bebe8 ("tty: don't panic on OOM in tty_set_ldisc()")
>>
>> from the tty.current tree and commit:
>>
>>   71472fa9c52b ("tty: Fix ldisc crash on reopened tty")
>>
>> from the tty tree.
>>
>> I fixed it up (see below) and can carry the fix as necessary. This
>> is now fixed as far as linux-next is concerned, but any non trivial
>> conflicts should be mentioned to your upstream maintainer when your tree
>> is submitted for merging.  You may also want to consider cooperating
>> with the maintainer of the conflicting tree to minimise any particularly
>> complex conflicts.
>>
>> --
>> Cheers,
>> Stephen Rothwell
>>
>> diff --cc drivers/tty/tty_ldisc.c
>> index b0500a0a87b8,4ee7742dced3..000000000000
>> --- a/drivers/tty/tty_ldisc.c
>> +++ b/drivers/tty/tty_ldisc.c
>> @@@ -621,14 -669,17 +621,15 @@@ int tty_ldisc_reinit(struct tty_struct
>>                 tty_ldisc_put(tty->ldisc);
>>         }
>>
>> -       /* switch the line discipline */
>> -       tty->ldisc = ld;
>>         tty_set_termios_ldisc(tty, disc);
>> -       retval = tty_ldisc_open(tty, tty->ldisc);
>> +       retval = tty_ldisc_open(tty, ld);
>>         if (retval) {
>> -               tty_ldisc_put(tty->ldisc);
>> -               tty->ldisc = NULL;
>>  -              if (!WARN_ON(disc == N_TTY)) {
>>  -                      tty_ldisc_put(ld);
>>  -                      ld = NULL;
>>  -              }
>> ++              tty_ldisc_put(ld);
>> ++              ld = NULL;
>>         }
>> +
>> +       /* switch the line discipline */
>> +       smp_store_release(&tty->ldisc, ld);
>>         return retval;
>>   }
>>
>
>
> Peter,
>
> Looking at your patch "tty: Fix ldisc crash on reopened tty", I think
> there is a missed barrier in tty_ldisc_ref. A single barrier does not
> have any effect, they always need to be in pairs. So I think we also
> need at least:
>
> @@ -295,7 +295,8 @@ struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
>         struct tty_ldisc *ld = NULL;
>
>         if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
> -               ld = tty->ldisc;
> +               ld = READ_ONCE(tty->ldisc);
> +               read_barrier_depends();
>                 if (!ld)
>                         ldsem_up_read(&tty->ldisc_sem);
>         }
>
>
> Or simply:
>
> @@ -295,7 +295,8 @@ struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
>         struct tty_ldisc *ld = NULL;
>
>         if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
> -               ld = tty->ldisc;
> +               /* pairs with smp_store_release in tty_ldisc_reinit */
> +               ld = smp_load_acquire(&tty->ldisc);
>                 if (!ld)
>                         ldsem_up_read(&tty->ldisc_sem);
>         }




I am also surprised that callers of tty_ldisc_reinit don't hold
ldisc_sem. I thought that ldisc_sem is what's supposed to protect
changes to ldisc. That would also auto fix the crash without any
tricky barriers as flush_to_ldisc uses tty_ldisc_ref.