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Subject[PATCH v2] n_tty: wake up poll(POLLRDNORM) on receiving data
Date
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".

Signed-off-by: Kosuke Tatsukawa <tatsu-ab1@nec.com>
Fixes: 57087d515441 ("tty: Fix spurious poll() wakeups")
Cc: stable@vger.kernel.org
---
drivers/tty/n_tty.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 5be6d02..b2b98fe 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1369,7 +1369,7 @@ static void n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)
put_tty_queue(c, ldata);
smp_store_release(&ldata->canon_head, ldata->read_head);
kill_fasync(&tty->fasync, SIGIO, POLL_IN);
- wake_up_interruptible_poll(&tty->read_wait, EPOLLIN);
+ wake_up_interruptible_poll(&tty->read_wait, EPOLLIN | EPOLLRDNORM);
return;
}
}
@@ -1589,7 +1589,7 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,

if (read_cnt(ldata)) {
kill_fasync(&tty->fasync, SIGIO, POLL_IN);
- wake_up_interruptible_poll(&tty->read_wait, EPOLLIN);
+ wake_up_interruptible_poll(&tty->read_wait, EPOLLIN | EPOLLRDNORM);
}
}
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