Public bug reported:
With linux kernel <=4.15.0-43 I can start reading serial device (TTY) in
block mode and write from another process at the same time
(simultaneously). After update to 4.15.0-45 I can't.
[Steps to reproduce]
1) Run in terminal:
cat /dev/ttyS0
"cat" must block terminal and wait for data.
2) Run in another terminal:
echo > /dev/ttyS0
Expected:
"cat" continue wait data and not exit.
Actual:
"cat" exit with return code "1" and show error -EAGAIN (-11):
"cat: /dev/ttyS0: Resource temporarily unavailable"
I think it's regression in kernel, due to new ldisc checks in TTY driver.
Please see:
git diff Ubuntu-hwe-4.15.0-43.46_16.04.1 Ubuntu-hwe-4.15.0-45.48_16.04.1 --
drivers/tty/tty_ldisc.c
git diff Ubuntu-hwe-4.15.0-43.46_16.04.1 Ubuntu-hwe-4.15.0-45.48_16.04.1 --
drivers/tty/n_tty.c
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 0fd18f5..48fe8d9 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -340,6 +340,11 @@ int tty_ldisc_lock(struct tty_struct *tty, unsigned long
timeout)
{
int ret;
+ /* Kindly asking blocked readers to release the read side */
+ set_bit(TTY_LDISC_CHANGING, &tty->flags);
+ wake_up_interruptible_all(&tty->read_wait);
+ wake_up_interruptible_all(&tty->write_wait);
+
ret = __tty_ldisc_lock(tty, timeout);
if (!ret)
return -EBUSY;
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 63c593a..9805ac1 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2188,7 +2197,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct
file *file,
break;
if (!timeout)
break;
- if (file->f_flags & O_NONBLOCK) {
+ if (tty_io_nonblock(tty, file)) {
retval = -EAGAIN;
break;
}
@@ -2342,7 +2351,7 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct
file *file,
}
if (!nr)
break;
- if (file->f_flags & O_NONBLOCK) {
+ if (tty_io_nonblock(tty, file)) {
retval = -EAGAIN;
break;
}
ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.15.0-45-generic 4.15.0-45.48~16.04.1
ProcVersionSignature: Ubuntu 4.15.0-45.48~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-45-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CurrentDesktop: MATE
Date: Thu Feb 7 11:42:43 2019
InstallationDate: Installed on 2017-08-01 (554 days ago)
InstallationMedia: Ubuntu 16.04.2 LTS "Xenial Xerus" - Release amd64
(20170215.2)
SourcePackage: linux-signed-hwe
UpgradeStatus: No upgrade log present (probably fresh install)
** Affects: linux-signed-hwe (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug xenial
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https://bugs.launchpad.net/bugs/1815021
Title:
Can't read and write simultaneously from serial port
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