zram: fix race between backing_dev_show and backing_dev_store

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);

gets the value of zram->backing_dev too early in backing_dev_show, which
resultin the value being NULL at the beginning, and not NULL later.

backtrace:
  d_path+0xcc/0x174
  file_path+0x10/0x18
  backing_dev_show+0x40/0xb4
  dev_attr_show+0x20/0x54
  sysfs_kf_seq_show+0x9c/0x10c
  kernfs_seq_show+0x28/0x30
  seq_read+0x184/0x488
  kernfs_fop_read+0x5c/0x1a4
  __vfs_read+0x44/0x128
  vfs_read+0xa0/0x138
  SyS_read+0x54/0xb4

Link: http://lkml.kernel.org/r/1571046839-16814-1-git-send-email-chenwandun@huawei.com
Signed-off-by: Chenwandun <chenwandun@huawei.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: <stable@vger.kernel.org>	[4.14+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index d58a359a6622..4285e75e52c3 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;