[PATCH] zram: fix race between backing_dev_show and backing_dev_store

[Chen Wandun]

From: Chenwandun <chenwandun@xxxxxxxxxx>

CPU0:				       CPU1:
backing_dev_show		       backing_dev_store
    ......				   ......
    file = zram->backing_dev;
    down_read(&zram->init_lock);	   down_read(&zram->init_init_lock)
    file_path(file, ...);		   zram->backing_dev = backing_dev;
    up_read(&zram->init_lock);		   up_read(&zram->init_lock);

get the value of zram->backing_dev too early in backing_dev_show,
that will result the value may be NULL at the begining, and not
NULL later.

backtrace:
[<ffffff8570e0f3ec>] d_path+0xcc/0x174
[<ffffff8570decd90>] file_path+0x10/0x18
[<ffffff85712f7630>] backing_dev_show+0x40/0xb4
[<ffffff85712c776c>] dev_attr_show+0x20/0x54
[<ffffff8570e835e4>] sysfs_kf_seq_show+0x9c/0x10c
[<ffffff8570e82b98>] kernfs_seq_show+0x28/0x30
[<ffffff8570e1c580>] seq_read+0x184/0x488
[<ffffff8570e81ec4>] kernfs_fop_read+0x5c/0x1a4
[<ffffff8570dee0fc>] __vfs_read+0x44/0x128
[<ffffff8570dee310>] vfs_read+0xa0/0x138
[<ffffff8570dee860>] SyS_read+0x54/0xb4

Signed-off-by: Chenwandun <chenwandun@xxxxxxxxxx>
---
 drivers/block/zram/zram_drv.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index d58a359..4285e75 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -413,13 +413,14 @@ static void reset_bdev(struct zram *zram)
 static ssize_t backing_dev_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
+	struct file *file;
 	struct zram *zram = dev_to_zram(dev);
-	struct file *file = zram->backing_dev;
 	char *p;
 	ssize_t ret;
 
 	down_read(&zram->init_lock);
-	if (!zram->backing_dev) {
+	file = zram->backing_dev;
+	if (!file) {
 		memcpy(buf, "none\n", 5);
 		up_read(&zram->init_lock);
 		return 5;
-- 
2.7.4