zram: close race by open overriding

[ Original patch from Minchan Kim <minchan@kernel.org> ]

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.

[sergey.senozhatsky@gmail.com: simplify reset_store()]
Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index a52a819..6a3b36d 100644 (file)
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1070,45 +1070,60 @@ static ssize_t reset_store(struct device *dev,
        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;
-
-       if (!do_reset) {
-               ret = -EINVAL;
-               goto out;
-       }
+       /* From now on, anyone can't open /dev/zram[0-9] */
+       zram->claim = true;
+       mutex_unlock(&bdev->bd_mutex);
 
-       /* 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);
 
+       mutex_lock(&bdev->bd_mutex);
+       zram->claim = false;
+       mutex_unlock(&bdev->bd_mutex);
+
        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;
 
-out:
-       mutex_unlock(&bdev->bd_mutex);
-       bdput(bdev);
        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

diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 042994e..6dbe2df 100644 (file)
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -115,5 +115,9 @@ struct zram {
         */
        u64 disksize;   /* bytes */
        char compressor[10];
+       /*
+        * zram is claimed so open request will be failed
+        */
+       bool claim; /* Protected by bdev->bd_mutex */
 };
 #endif