linux-kernel - Re: [PATCH v2 08/11] tty: splice

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Date: Wed, 3 Jan 2024 12:36:46 +0100
From: Jiri Slaby <jirislaby@...nel.org>
To: Ahelenia Ziemiańska <nabijaczleweli@...ijaczleweli.xyz>,
 "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Cc: Jens Axboe <axboe@...nel.dk>, Christian Brauner <brauner@...nel.org>,
 Alexander Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v2 08/11] tty: splice_read: disable

On 21. 12. 23, 4:09, Ahelenia Ziemiańska wrote:
> We request non-blocking I/O in the generic copy_splice_read, but
> "the tty layer doesn't actually honor the IOCB_NOWAIT flag for
> various historical reasons.". This means that a tty->pipe splice
> will happily sleep with the pipe locked forever, and any process
> trying to take it (due to an open/read/write/&c.) will enter
> uninterruptible sleep.
> 
> This also masks inconsistent wake-ups (usually every second line)
> when splicing from ttys in icanon mode.
> 
> Link: https://lore.kernel.org/linux-fsdevel/CAHk-=wimmqG_wvSRtMiKPeGGDL816n65u=Mq2+H3-=uM2U6FmA@mail.gmail.com/
> Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@...ijaczleweli.xyz>
> ---
>   drivers/tty/tty_io.c | 2 --
>   1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 06414e43e0b5..50c2957a9c7f 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -465,7 +465,6 @@ static const struct file_operations tty_fops = {
>   	.llseek		= no_llseek,
>   	.read_iter	= tty_read,
>   	.write_iter	= tty_write,
> -	.splice_read	= copy_splice_read,
>   	.splice_write	= iter_file_splice_write,

This and the other patch effectively reverts dd78b0c483e33 and 
9bb48c82aced0. I.e. it breaks "things". Especially:

commit 9bb48c82aced07698a2d08ee0f1475a6c4f6b266
Author: Linus Torvalds <torvalds@...ux-foundation.org>
Date:   Tue Jan 19 11:41:16 2021 -0800

     tty: implement write_iter

     This makes the tty layer use the .write_iter() function instead of the
     traditional .write() functionality.

     That allows writev(), but more importantly also makes it possible to
     enable .splice_write() for ttys, reinstating the "splice to tty"
     functionality that was lost in commit 36e2c7421f02 ("fs: don't allow
     splice read/write without explicit ops").

     Fixes: 36e2c7421f02 ("fs: don't allow splice read/write without 
explicit ops")


What are those "things" doing that "splice to tty", I don't recall and 
the commit message above ^^^ does not spell that out. Linus?

thanks,
-- 
js