linux-kernel - [PATCH 3.2 31/94] hvc: ensure hvc_init is only ever called once in hvc

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Message-ID: <lsq.1398647482.924871999@decadent.org.uk>
Date:	Mon, 28 Apr 2014 02:11:22 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Jim Somerville" <Jim.Somerville@...driver.com>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	"Rusty Russell" <rusty@...tcorp.com.au>,
	"Paul Gortmaker" <paul.gortmaker@...driver.com>
Subject: [PATCH 3.2 31/94] hvc: ensure hvc_init is only ever called once
 in hvc_console.c

3.2.58-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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

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@...tcorp.com.au>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Fixes: 3e6c6f630a52 ("Delay creation of khcvd thread")
Reported-by: Jim Somerville <Jim.Somerville@...driver.com>
Tested-by: Jim Somerville <Jim.Somerville@...driver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/tty/hvc/hvc_console.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- 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
@@ -825,7 +829,7 @@ struct hvc_struct *hvc_alloc(uint32_t vt
 	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);

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