linux-kernel - Re: [PATCH] n_tty: wake up poll(POLLRDNORM) on receiving data

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Message-ID: <TYCPR01MB81901234932D9FD1327DF093A5209@TYCPR01MB8190.jpnprd01.prod.outlook.com>
Date:   Wed, 26 Jan 2022 23:34:39 +0000
From:   TATSUKAWA KOSUKE(立川　江介) 
        <tatsu-ab1@....com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Jiri Slaby <jirislaby@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] n_tty: wake up poll(POLLRDNORM) on receiving data 

Hi, thank you for the reply.

> On Mon, Jan 24, 2022 at 08:17:22AM +0000, TATSUKAWA KOSUKE(立川 江介) wrote:
> > Event POLLRDNORM should be equivalent to POLLIN when used as event in
> > poll().
> 
> Where is that documented?

It's in "man poll".  Below is an excerpt from the man page.
              POLLRDNORM
                     Equivalent to POLLIN.

I'll send a v2 patch in a separate message.

Many other calls to wake_up_interruptible_poll() in the kernel source
code already specify "POLLIN | POLLRDNORM", but there are places under
drivers/tty which only passes POLLIN (i.e. pty.c, tty_io.c,
tty_ldisc.c).  I haven't yet made test programs to hit the other
locations and I wasn't sure if they should also be fixed just for
consistency.

Best regards.


> > However, in n_tty driver, POLLRDNORM does not return until
> > timeout even if there is terminal input, whereas POLLIN returns.
> > 
> > The following test program works until kernel-3.17, but the test stops
> > in poll() after commit 57087d515441 ("tty: Fix spurious poll() wakeups").
> > 
> > [Steps to run test program]
> >   $ cc -o test-pollrdnorm test-pollrdnorm.c
> >   $ ./test-pollrdnorm
> >   foo          <-- Type in something from the terminal followed by [RET].
> >                    The string should be echoed back.
> > 
> >   ------------------------< test-pollrdnorm.c >------------------------
> >   #include <stdio.h>
> >   #include <errno.h>
> >   #include <poll.h>
> >   #include <unistd.h>
> > 
> >   void main(void)
> >   {
> > 	int		n;
> > 	unsigned char	buf[8];
> > 	struct pollfd	fds[1] = {{ 0, POLLRDNORM, 0 }};
> > 
> > 	n = poll(fds, 1, -1);
> > 	if (n < 0)
> > 		perror("poll");
> > 	n = read(0, buf, 8);
> > 	if (n < 0)
> > 		perror("read");
> > 	if (n > 0)
> > 		write(1, buf, n);
> >   }
> >   ------------------------------------------------------------------------
> > 
> > The attached patch fixes this problem.
> > 
> > Signed-off-by: Kosuke Tatsukawa <tatsu-ab1@....com>
> > Fixes: commit 57087d515441 ("tty: Fix spurious poll() wakeups")
> 
> No need for "commit" here, please remove as the documentation asks you
> to.
> 
> Can you resend this with this fixed up, and a pointer to where the
> documentation is for this functionality.
> 
> thanks,
> 
> greg k-h
---
Kosuke TATSUKAWA  | 1st Platform Software Division
                  | NEC Solution Innovators
                  | tatsu-ab1@....com