CVS commit: src/sys

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

Module Name:    src
Committed By:   riastradh
Date:           Mon May 22 14:58:22 UTC 2023

Modified Files:
        src/sys/kern: subr_autoconf.c
        src/sys/sys: device.h

Log Message:
autoconf(9): New functions for referenced attach/detach.

New functions:

- config_found_acquire(dev, aux, print, cfargs)
- config_attach_acquire(parent, cf, aux, print, cfargs)
- config_attach_pseudo_acquire(cf, aux)
- config_detach_release(dev, flags)
- device_acquire(dev)

The config_*_acquire functions are like the non-acquire versions, but
they return a referenced device_t, which is guaranteed to be safe to
use until released.  The device's detach function may run while it is
referenced, but the device_t will not be freed and the parent's
.ca_childdetached routine will not be called.

=> config_attach_pseudo_acquire additionally lets you pass an aux
   argument to the device's .ca_attach routine, unlike
   config_attach_pseudo which always passes NULL.

=> Eventually, config_found, config_attach, and config_attach_pseudo
   should be made to return void, because use of the device_t they
   return is unsafe without the kernel lock and difficult to use
   safely even with the kernel lock or in a UP system.  For now,
   they require the caller to hold the kernel lock, while
   config_*_acquire do not.

config_detach_release is like device_release and then config_detach,
but avoids the race inherent with that sequence.

=> Eventually, config_detach should be eliminated, because getting at
   the device_t it needs is unsafe without the kernel lock and
   difficult to use safely even with the kernel lock or in a UP
   system.  For now, it requires the caller to hold the kernel lock,
   while config_detach_release does not.

device_acquire acquires a reference to a device.  It never fails and
can be used in thread context (but not interrupt context, hard or
soft).  Caller is responsible for ensuring that the device_t cannot
be freed; in other words, the device_t must be made unavailable to
any device_acquire callers before the .ca_detach function returns.
Typically device_acquire will be used in a read section (mutex,
rwlock, pserialize, &c.) in a data structure lookup, with
corresponding logic in the .ca_detach function to remove the device
from the data structure and wait for all read sections to complete.

Proposed on tech-kern:
https://mail-index.netbsd.org/tech-kern/2023/05/10/msg028889.html


To generate a diff of this commit:
cvs rdiff -u -r1.310 -r1.311 src/sys/kern/subr_autoconf.c
cvs rdiff -u -r1.185 -r1.186 src/sys/sys/device.h

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