FAILED: patch "[PATCH] zram: fix race between backing_dev_show and backing_dev_store" failed to apply to 4.14-stable tree

The patch below does not apply to the 4.14-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f7daefe4231e57381d92c2e2ad905a899c28e402 Mon Sep 17 00:00:00 2001
From: Chenwandun <[email protected]>
Date: Fri, 18 Oct 2019 20:20:14 -0700
Subject: [PATCH] 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

index d58a359a6622..4285e75e52c3 100644
@@ -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;