[PATCH 3.12 100/182] hvc: ensure hvc_init is only ever called once in hvc_console.c

Jiri Slaby
2014-05-13T10:35:22Z

From: Paul Gortmaker <paul.gortmaker@windriver.com>

3.12-stable review patch. If anyone has any objections, please let me know.

===============

commit f76a1cbed18c86e2d192455f0daebb48458965f3 upstream.

Commit 3e6c6f630a5282df8f3393a59f10eb9c56536d23 ("Delay creation of
khcvd thread") moved the call of hvc_init from being a device_initcall
into hvc_alloc, and used a non-null hvc_driver as indication of whether
hvc_init had already been called.

The problem with this is that hvc_driver is only assigned a value
at the bottom of hvc_init, and so there is a window where multiple
hvc_alloc calls can be in progress at the same time and hence try
and call hvc_init multiple times. Previously the use of device_init
guaranteed that hvc_init was only called once.

This manifests itself as sporadic instances of two hvc_init calls
racing each other, and with the loser of the race getting -EBUSY
from tty_register_driver() and hence that virtual console fails:

Couldn't register hvc console driver
virtio-ports vport0p1: error -16 allocating hvc for port

Here we add an atomic_t to guarantee we'll never run hvc_init twice.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: 3e6c6f630a52 ("Delay creation of khcvd thread")
Reported-by: Jim Somerville <Jim.Somerville@windriver.com>
Tested-by: Jim Somerville <Jim.Somerville@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
drivers/tty/hvc/hvc_console.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index 9eba119bcdd3..620da5f88e91 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -31,6 +31,7 @@
#include <linux/list.h>
#include <linux/module.h>
#include <linux/major.h>
+#include <linux/atomic.h>
#include <linux/sysrq.h>
#include <linux/tty.h>
#include <linux/tty_flip.h>
@@ -70,6 +71,9 @@ static struct task_struct *hvc_task;
/* Picks up late kicks after list walk but before schedule() */
static int hvc_kicked;

+/* hvc_init is triggered from hvc_alloc, i.e. only when actually used */
+static atomic_t hvc_needs_init __read_mostly = ATOMIC_INIT(-1);
+
static int hvc_init(void);

#ifdef CONFIG_MAGIC_SYSRQ
@@ -851,7 +855,7 @@ struct hvc_struct *hvc_alloc(uint32_t vtermno, int data,
int i;

/* We wait until a driver actually comes along */
- if (!hvc_driver) {
+ if (atomic_inc_not_zero(&hvc_needs_init)) {
int err = hvc_init();
if (err)
return ERR_PTR(err);
--
1.9.3