Hi,
In __blk_mq_update_nr_hw_queues(), we don't check the return value of
blk_mq_sysfs_register_hctxs(). When sysfs creation fails, there's no
proper error handling. This leads to a kernel warning during subsequent
__blk_mq_update_nr_hw_queues() calls or disk removal:
```
kernfs: can not remove 'nr_tags', no directory
WARNING: CPU: 2 PID: 805 at fs/kernfs/dir.c:1703
kernfs_remove_by_name_ns+0x12e/0x140
Call Trace:
<TASK>
remove_files+0x39/0xb0
sysfs_remove_group+0x48/0xf0
sysfs_remove_groups+0x31/0x60
__kobject_del+0x23/0xf0
kobject_del+0x17/0x40
blk_mq_unregister_hctx+0x5d/0x80
blk_mq_sysfs_unregister_hctxs+0x89/0xd0
blk_mq_update_nr_hw_queues+0x31c/0x820
nullb_update_nr_hw_queues+0x71/0xe0 [null_blk]
nullb_device_submit_queues_store+0xa4/0x130 [null_blk]
```
Should we add error checking for blk_mq_sysfs_register_hctxs() and
propagate the error to abort the update operation when it fails? This
would prevent subsequent operations from hitting invalid sysfs entries.
--
Thanks,
Nan
