illumos gate

#6874 introduced the regression

From the illumos-list:

Me:

Hi.

We found that ZFS_IOC_USERSPACE_MANY breaks final-stage of zfs-recv
where ZFS does clone-swap.

"6874 rollback and receive need to reset ZPL state to what's on disk" 
introduces some changes that causes the behavior.

ZFS_IOC_USERSPACE_MANY -> zfs_ioc_userspace_many() -> zfsvfs_hold() ->
zfsvfs_create() -> ... -> dsl_dataset_long_hold()

dsl_dataset_clone_swap_check_impl() -> dsl_dataset_handoff_check() ->
EBUSY if there are long holds

This affect only non-mounted FS.

Matthew Ahrens:

I think your analysis is correct.  There are a few other places where we also abuse the long hold if the fs is not mounted: zfs_ioc_userspace_one, zfs_prop_set_special(VERSION or userquota).

These are abuses of the long-hold mechanism because they aren't really holding for a long time.  We would rather that the receive code wait for them to complete.

I assume that "breaks" here means that the receive can fail if you do a concurrent "zfs userspace" (or other ZFS_IOC_USERSPACE* ioctl).  That definitely seems like undesirable behavior.  Could you file a bug?  But FYI, I'm not sure when I personally will be able to get to it.

--matt