This is required to test filesystems with a non-trivial implementation of
FilesystemImpl.Release(). Propagation isn't handled yet, and umount isn't yet
plumbed out to VirtualFilesystem.UmountAt(), but otherwise the implementation
of umount is believed to be correct.
- Move entering mountTable.seq writer critical sections to callers of
mountTable.{insert,remove}Seqed. This is required since umount(2) must ensure
that no new references are taken on the candidate mount after checking that
it isn't busy, which is only possible by entering a vfs.mountTable.seq writer
critical section before the check and remaining in it until after
VFS.umountRecursiveLocked() is complete. (Linux does the same thing:
fs/namespace.c:do_umount() => lock_mount_hash(),
fs/pnode.c:propagate_mount_busy(), umount_tree(), unlock_mount_hash().)
- It's not possible for dentry deletion to umount while only holding
VFS.mountMu for reading, but it's also very unappealing to hold VFS.mountMu
exclusively around e.g. gofer unlink RPCs. Introduce dentry.mu to avoid these
problems. This means that VFS.mountMu is never acquired for reading, so
change it to a sync.Mutex.
PiperOrigin-RevId: 282444343
128948d6ae