---
- branch: MAIN
  date: Mon Mar 28 12:33:32 UTC 2022
  files:
  - new: '1.40'
    old: '1.39'
    path: src/sys/kern/subr_devsw.c
    pathrev: src/sys/kern/subr_devsw.c@1.40
    type: modified
  id: 20220328T123332Z.38a3066d21c54c305ead16d9c6bac46f560f0ce2
  log: |
    driver(9): Fix synchronization of devsw_attach/lookup/detach.

    (`dev' means either `bdev' or `cdev' for brevity here, e.g. in
    `devsw_lookup' (bdevsw_lookup, cdevsw_lookup), `dev_open' (bdev_open,
    cdev_open), `maxdevsws', &c., except for `devsw_attach' and
    `devsw_detach' which are taken literally.)

    - Use atomic_store_release and atomic_load_consume for devsw and
      tables and their entries, which are read unlocked and thus require
      memory barriers to ensure ordering between initialization in
      devsw_attach and use in dev_lookup.

    - Use pserialize(9) and localcount(9) to synchronize dev_open and
      devsw_detach.

      => Driver must ensure d_open fails and all open instances have been
         closed by the time it calls devsw_detach.

      => Bonus: dev_open is no longer globally serialized through
         device_lock.

    - Use atomic_store_release and atomic_load_acquire for max_devsws,
      which is used in conditionals in the new devsw_lookup_acquire.

      => It is safe to use atomic_load_relaxed in devsw_lookup because
         the caller must guarantee the entry is stable, so any increase
         of max_devsws must have already happened.

      => devsw_lookup and devsw_lookup_acquire assume that max_devsws
         never goes down.  If you change this you must find some way to
         adapt the users, preferably without adding much overhead so that
         devsw operations are cheap.

    This change introduces an auxiliary table devswref mapping device
    majors to localcounts of opens in progress.  The auxiliary table only
    occupies one pointer's worth of memory in a monolithic kernel, and is
    allocated on the fly for dynamically loaded modules.  We could ask
    the module itself to reserve storage for it, but I don't see much
    value in that, and it would require some changes to the ABI and to
    config(8).

    - Omit needless boolean indirection.
  module: src
  subject: 'CVS commit: src/sys/kern'
  unixtime: '1648470812'
  user: riastradh