---
- branch: MAIN
  date: Mon Mar 28 12:37:56 UTC 2022
  files:
  - new: '1.141'
    old: '1.140'
    path: src/sys/kern/vfs_vnode.c
    pathrev: src/sys/kern/vfs_vnode.c@1.141
    type: modified
  - new: '1.209'
    old: '1.208'
    path: src/sys/miscfs/specfs/spec_vnops.c
    pathrev: src/sys/miscfs/specfs/spec_vnops.c@1.209
    type: modified
  id: 20220328T123756Z.9682b639ec962d4e204b41d4b884a1d88da1390a
  log: |
    specfs: Remove specnode from hash table in spec_node_revoke.

    Previously, it was possible for spec_node_lookup_by_dev to handle a
    speconde that a concurrent spec_node_destroy is about to remove from
    the hash table and then free, as soon as spec_node_lookup_by_dev
    releases device_lock.

    Now, the ordering is:

    1. Remove specnode from hash table in spec_node_revoke.  At this
       point, no _new_ vnode references are possible (other than possibly
       one acquired by vcache_vget under v_interlock), but there may be
       existing ones.

    2. Mark vnode reclaimed so vcache_vget will fail.

    3. The last vrele (or equivalent logic in vcache_vget) will then free
       the specnode in spec_node_destroy.

    This way, _if_ a thread in spec_node_lookup_by_dev finds a specnode
    in the hash table under device_lock/v_interlock, _then_ it will not
    be freed until the thread completes vcache_vget.

    This change requires calling spec_node_revoke unconditionally for
    device special nodes, not just for active ones.  Might introduce
    slightly more contention on device_lock but not much because we
    already have to take it in this path anyway a little later in
    spec_node_destroy.
  module: src
  subject: 'CVS commit: src/sys'
  unixtime: '1648471076'
  user: riastradh