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Message-ID: <CAHk-=winHP_cXTJvJzeudgq-xCnWQPmVp0_O6wT9nFsFNQYrkQ@mail.gmail.com>
Date: Mon, 6 Dec 2021 10:07:30 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jirislaby@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
syzbot+5f47a8cea6a12b77a876@...kaller.appspotmail.com,
Marco Elver <elver@...gle.com>,
Max Filippov <jcmvbkbc@...il.com>,
David Sterba <dsterba@...e.com>,
Bhaskar Chowdhury <unixbhaskar@...il.com>,
nick black <dankamongmen@...il.com>,
Igor Matheus Andrade Torrente <igormtorrente@...il.com>
Subject: Re: [PATCH] tty: n_hdlc: make n_hdlc_tty_wakeup() asynchronous
On Mon, Dec 6, 2021 at 3:45 AM Tetsuo Handa
<penguin-kernel@...ove.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
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