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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 6 Dec 2021 10:07:30 -0800
Message-ID: <CAHk-=winHP_cXTJvJzeudgq-xCnWQPmVp0_O6wT9nFsFNQYrkQ@mail.gmail.com>
Subject: Re: [PATCH] tty: n_hdlc: make n_hdlc_tty_wakeup() asynchronous
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        syzbot+5f47a8cea6a12b77a876@syzkaller.appspotmail.com,
        Marco Elver <elver@google.com>,
        Max Filippov <jcmvbkbc@gmail.com>,
        David Sterba <dsterba@suse.com>,
        Bhaskar Chowdhury <unixbhaskar@gmail.com>,
        nick black <dankamongmen@gmail.com>,
        Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
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On Mon, Dec 6, 2021 at 3:45 AM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> Linus suspected that "struct tty_ldisc"->ops->write_wakeup() must not
> sleep, and Jiri confirmed it from include/linux/tty_ldisc.h. Thus, defer
> n_hdlc_send_frames() from n_hdlc_tty_wakeup() to a WQ context like
> net/nfc/nci/uart.c does.

Thanks, this looks good to me.

That said, I think there's pretty much the *exact* same pattern in

    drivers/net/caif/caif_serial.c:
        write_wakeup() causes "handle_tx()", which then calls tty->ops->write().

    drivers/net/hamradio/mkiss.c
        mkiss_write_wakeup() -> tty->ops->write()

    drivers/tty/n_gsm.c:
        gsmld_write_wakeup -> gsm_data_kick() -> gsmld_output ->
gsm->tty->ops->write()

so this does seem to be a common bug pattern for code that has never
really seen a lot of testing.

The core tty stuff seems to get it right, but maybe I missed something
in my quick "grep and look for patterns".

So I think this patch is good, but I do wonder if perhaps we should
move the "work_struct" into the tty layer itself, and do the whole
"schedule_work()" at that level.

Some code never wants it (most notably the regular n_tty one), but at
least n_tty doesn't really care, I suspect. n_tty is using
write_wakeup() literally just for fasync handling, so I suspect it's
not exactly going to be performance-critical.

Of course, maybe the fix is to just fix caif_serial/mkiss and n_gsm.
Or mark them broken - does anybody use them?

              Linus

             Linus