Re: [PATCH V1 1/1] Fix race between iscsi logout and systemd-udevd

Ming Lei
2021-05-12T07:23:32Z

On Wed, May 12, 2021 at 08:35:05AM +0200, Christoph Hellwig wrote:
> On Wed, May 12, 2021 at 11:23:59AM +0800, Ming Lei wrote:
> >
> > 1) code path BLKRRPART:
> > mutex_lock(bdev->bd_mutex)
> > down_read(&bdev_lookup_sem);
> >
> > 2) del_gendisk():
> > down_write(&bdev_lookup_sem);
> > mutex_lock(&disk->part0->bd_mutex);
> >
> > Given GENHD_FL_UP is only checked when opening one bdev, and
> > fsync_bdev() and __invalidate_device() needn't to open bdev, so
> > the following way may work for your issue:
>
> If we move the clearing of GENHD_FL_UP earlier we can do away with
> bdev_lookup_sem entirely I think. Something like this untested patch:
>
> diff --git a/block/genhd.c b/block/genhd.c
> index a5847560719c..ef717084b343 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -29,8 +29,6 @@
>
> static struct kobject *block_depr;
>
> -DECLARE_RWSEM(bdev_lookup_sem);
> -
> /* for extended dynamic devt allocation, currently only one major is used */
> #define NR_EXT_DEVT (1 << MINORBITS)
> static DEFINE_IDA(ext_devt_ida);
> @@ -609,13 +607,8 @@ void del_gendisk(struct gendisk *disk)
> blk_integrity_del(disk);
> disk_del_events(disk);
>
> - /*
> - * Block lookups of the disk until all bdevs are unhashed and the
> - * disk is marked as dead (GENHD_FL_UP cleared).
> - */
> - down_write(&bdev_lookup_sem);
> -
> mutex_lock(&disk->open_mutex);
> + disk->flags &= ~GENHD_FL_UP;
> blk_drop_partitions(disk);
> mutex_unlock(&disk->open_mutex);
>
> @@ -627,10 +620,7 @@ void del_gendisk(struct gendisk *disk)
> * up any more even if openers still hold references to it.
> */
> remove_inode_hash(disk->part0->bd_inode);
> -
> set_capacity(disk, 0);
> - disk->flags &= ~GENHD_FL_UP;
> - up_write(&bdev_lookup_sem);
>
> if (!(disk->flags & GENHD_FL_HIDDEN)) {
> sysfs_remove_link(&disk_to_dev(disk)->kobj, "bdi");
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 8dd8e2fd1401..bde23940190f 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1377,33 +1377,24 @@ struct block_device *blkdev_get_no_open(dev_t dev)
> struct block_device *bdev;
> struct gendisk *disk;
>
> - down_read(&bdev_lookup_sem);
> bdev = bdget(dev);
> if (!bdev) {
> - up_read(&bdev_lookup_sem);
> blk_request_module(dev);
> - down_read(&bdev_lookup_sem);
> -
> bdev = bdget(dev);
> if (!bdev)
> - goto unlock;
> + return NULL;
> }
>
> disk = bdev->bd_disk;
> if (!kobject_get_unless_zero(&disk_to_dev(disk)->kobj))
> goto bdput;
> - if ((disk->flags & (GENHD_FL_UP | GENHD_FL_HIDDEN)) != GENHD_FL_UP)
> - goto put_disk;
> if (!try_module_get(bdev->bd_disk->fops->owner))
> goto put_disk;
> - up_read(&bdev_lookup_sem);
> return bdev;
> put_disk:
> put_disk(disk);
> bdput:
> bdput(bdev);
> -unlock:
> - up_read(&bdev_lookup_sem);
> return NULL;
> }
>
> @@ -1462,7 +1453,10 @@ struct block_device *blkdev_get_by_dev(dev_t dev, fmode_t mode, void *holder)
>
> disk_block_events(disk);
>
> + ret = -ENXIO;
> mutex_lock(&disk->open_mutex);
> + if ((disk->flags & (GENHD_FL_UP | GENHD_FL_HIDDEN)) != GENHD_FL_UP)
> + goto abort_claiming;
> if (bdev_is_partition(bdev))
> ret = blkdev_get_part(bdev, mode);
> else

This patch looks fine, and new openers can be prevented really with help
of ->open_mutex.

Thanks,
Ming