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This count is memoized together with the lookup metadata in directory inode, and we assert that accesses to lookup metadata are done under the same lock generation as they were stored. Enabled under DIAGNOSTICS. UFS saves additional data for parent dirent when doing lookup (i_offset, i_count, i_endoff), and this data is used later by VOPs operating on dirents. If parent vnode exclusive lock is dropped and re-acquired between lookup and the VOP call, we corrupt directories. Framework asserts that corruption cannot occur that way, by tracking vnode lock generation counter. Updates to inode dirent members also save the counter, while users compare current and saved counters values. Also, fix a case in ufs_lookup_ino() where i_offset and i_count could be updated under shared lock. It is not a bug on its own since dvp i_offset results from such lookup cannot be used, but it causes false positive in the checker. In collaboration with: pho Reviewed by: mckusick (previous version), markj Tested by: markj (syzkaller), pho Sponsored by: The FreeBSD Foundation Differential revision: https://reviews.freebsd.org/D26136
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