CVS commit: src/sys/kern

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Thu Feb 23 02:58:28 UTC 2023

Modified Files:
        src/sys/kern: kern_descrip.c

Log Message:
kern_descrip.c: Fix membars around reference count decrement.

In general, the `last one out hit the lights' style of reference
counting (as opposed to the `whoever's destroying must wait for
pending users to finish' style) requires memory barriers like so:

        ... usage of resources associated with object ...
        membar_release();
        if (atomic_dec_uint_nv(&obj->refcnt) != 0)
                return;
        membar_acquire();
        ... freeing of resources associated with object ...

This way, all usage happens-before all freeing.  This fixes several
errors:

- fd_close failed to ensure whatever its caller did would
  happen-before the freeing, in the case where another thread is
  concurrently trying to close the fd (ff->ff_file == NULL).

  Fix: Add membar_release before atomic_dec_uint(&ff->ff_refcnt) in
  that branch.

- fd_close failed to ensure all loads its caller had issued will have
  happened-before the freeing, in the case where the fd is still in
  use by another thread (fdp->fd_refcnt > 1 and ff->ff_refcnt-- > 0).

  Fix: Change membar_producer to membar_release before
  atomic_dec_uint(&ff->ff_refcnt).

- fd_close failed to ensure that any usage of fp by other callers
  would happen-before any freeing it does.

  Fix: Add membar_acquire after atomic_dec_uint_nv(&ff->ff_refcnt).

- fd_free failed to ensure that any usage of fdp by other callers
  would happen-before any freeing it does.

  Fix: Add membar_acquire after atomic_dec_uint_nv(&fdp->fd_refcnt).

While here, change membar_exit -> membar_release.  No semantic
change, just updating away from the legacy API.

XXX pullup-8
XXX pullup-9
XXX pullup-10


To generate a diff of this commit:
cvs rdiff -u -r1.251 -r1.252 src/sys/kern/kern_descrip.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.