zram: close race by open overriding

[ Original patch from Minchan Kim <[e-mail]> ]

Commit ba6b17d68c8e ("zram: fix umount-reset_store-mount race
condition") introduced bdev->bd_mutex to protect a race between mount
and reset. At that time, we don't have dynamic zram-add/remove feature
so it was okay.

However, as we introduce dynamic device feature, bd_mutex became
trouble.

CPU 0

echo 1 > /sys/block/zram<id>/reset
-> kernfs->s_active(A)
-> zram:reset_store->bd_mutex(B)

CPU 1

echo <id> > /sys/class/zram/zram-remove
->zram:zram_remove: bd_mutex(B)
-> sysfs_remove_group
-> kernfs->s_active(A)

IOW, AB -> BA deadlock

The reason we are holding bd_mutex for zram_remove is to prevent
any incoming open /dev/zram[0-9]. Otherwise, we could remove zram
others already have opened. But it causes above deadlock problem.

To fix the problem, this patch overrides block_device.open and
it returns -EBUSY if zram asserts he claims zram to reset so any
incoming open will be failed so we don't need to hold bd_mutex
for zram_remove ayn more.

This patch is to prepare for zram-add/remove feature.

[[e-mail]: simplify reset_store()]
Signed-off-by: Minchan Kim <[e-mail]>
Acked-by: Sergey Senozhatsky <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/block/zram/zram_drv.c

···
 	struct zram *zram;
 	struct block_device *bdev;
 
+	ret = kstrtou16(buf, 10, &do_reset);
+	if (ret)
+		return ret;
+
+	if (!do_reset)
+		return -EINVAL;
+
 	zram = dev_to_zram(dev);
 	bdev = bdget_disk(zram->disk, 0);
-
 	if (!bdev)
 		return -ENOMEM;
 
 	mutex_lock(&bdev->bd_mutex);
-	/* Do not reset an active device! */
-	if (bdev->bd_openers) {
-		ret = -EBUSY;
-		goto out;
+	/* Do not reset an active device or claimed device */
+	if (bdev->bd_openers || zram->claim) {
+		mutex_unlock(&bdev->bd_mutex);
+		bdput(bdev);
+		return -EBUSY;
 	}
 
-	ret = kstrtou16(buf, 10, &do_reset);
-	if (ret)
-		goto out;
+	/* From now on, anyone can't open /dev/zram[0-9] */
+	zram->claim = true;
+	mutex_unlock(&bdev->bd_mutex);
 
-	if (!do_reset) {
-		ret = -EINVAL;
-		goto out;
-	}
-
-	/* Make sure all pending I/O is finished */
+	/* Make sure all the pending I/O are finished */
 	fsync_bdev(bdev);
 	zram_reset_device(zram);
-
-	mutex_unlock(&bdev->bd_mutex);
 	revalidate_disk(zram->disk);
 	bdput(bdev);
 
-	return len;
-
-out:
+	mutex_lock(&bdev->bd_mutex);
+	zram->claim = false;
 	mutex_unlock(&bdev->bd_mutex);
-	bdput(bdev);
+
+	return len;
+}
+
+static int zram_open(struct block_device *bdev, fmode_t mode)
+{
+	int ret = 0;
+	struct zram *zram;
+
+	WARN_ON(!mutex_is_locked(&bdev->bd_mutex));
+
+	zram = bdev->bd_disk->private_data;
+	/* zram was claimed to reset so open request fails */
+	if (zram->claim)
+		ret = -EBUSY;
+
 	return ret;
 }
 
 static const struct block_device_operations zram_devops = {
+	.open = zram_open,
 	.swap_slot_free_notify = zram_slot_free_notify,
 	.rw_page = zram_rw_page,
 	.owner = THIS_MODULE

drivers/block/zram/zram_drv.h

···
 	 */
 	u64 disksize;	/* bytes */
 	char compressor[10];
+	/*
+	 * zram is claimed so open request will be failed
+	 */
+	bool claim; /* Protected by bdev->bd_mutex */
 };
 #endif