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The poll man page says POLLRDNORM is equivalent to POLLIN when used as
an event.
$ man poll
<snip>
POLLRDNORM
Equivalent to POLLIN.
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. Many calls to
wake_up_interruptible_poll() in the kernel source code already specify
"POLLIN | POLLRDNORM".
Fixes: 57087d515441 ("tty: Fix spurious poll() wakeups")
Cc: stable@vger.kernel.org
Signed-off-by: Kosuke Tatsukawa <tatsu-ab1@nec.com>
Link: https://lore.kernel.org/r/TYCPR01MB81901C0F932203D30E452B3EA5209@TYCPR01MB8190.jpnprd01.prod.outlook.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
c816b2e65b