e881d8757c
On FreeBSD VFS_HOLD/VN_RELE were mapped to MNT_REF/MNT_REL that manipulate mnt_ref. But the job of properly maintaining the reference count is already automatically performed by insmntque(9) and delmntque(9). So, in effect all ZFS vnodes referenced the corresponding mountpoint twice. That was completely harmless, but we want to be very explicit about what FreeBSD VFS APIs are used, because illumos VFS_HOLD and FreeBSD MNT_REF provide quite different guarantees with respect to the held vfs_t / mountpoint. On illumos VFS_HOLD is sufficient to guarantee that vfs_t.vfs_data stays valid. On the other hand, on FreeBSD MNT_REF does *not* provide the same guarantee about mnt_data. We have to use vfs_busy() to get that guarantee. Thus, the calls to VFS_HOLD/VFS_RELE on vnode init and fini are removed. VFS_HOLD calls are replaced with vfs_busy in the ioctl handlers. And because vfs_busy has a richer interface that can not be dumbed down in all cases it's better to explicitly use it rather than trying to mask it behind VFS_HOLD. This change fixes a panic that could result from a race between zfs_umount() and zfs_ioc_rollback(). We observed a case where zfsvfs_free() tried to destroy data that zfsvfs_teardown() was still using. That happened because there was nothing to prevent unmounting of a ZFS filesystem that was in between zfs_suspend_fs() and zfs_resume_fs(). Reviewed by: kib, smh MFC after: 3 weeks Sponsored by: ClusterHQ Differential Revision: https://reviews.freebsd.org/D2794
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