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Scott James Remnant bb38678e3c udevadm: settle - synchronise with the udev daemon
There's still a slight race condition when using udevadm settle, if the
udev daemon has a pending inotify event but hasn't yet generated the
"change" uevent for it, the kernel and udev sequence numbers will match
and settle will exit.

Now udevadm settle will send a control message to udevd, which will
respond by sending SIGUSR1 back to the waiting udevadm settle once it
has completed the main loop iteration in which it received the control
message.

If there were no pending inotify events, this will simply wake up the
udev daemon and allow settle to continue.  If there are pending inotify
events, they are handled first in the main loop so when settle is
continued they will have been turned into uevents and the kernel
sequence number will have been incremented.

Since the inotify event is pending for udevd when the close() system
call returns (it's queued as part of the kernel handling for that system
call), and since the kernel sequence number is incremented by writing to
the uevent file (as udevd does), this solves the race.

When the settle continues, if there were pending inotify events that
udevd had not read, they are now pending uevents which settle can wait
for.

Signed-off-by: Scott James Remnant <scott@ubuntu.com>
2009-03-12 10:02:37 +00:00
docs/writing_udev_rules doc: writing udev rules - refer to 'udevadm info' instead of 'udevinfo' 2009-01-24 15:24:01 +01:00
extras volume_id: ntfs - fix uuid setting 2009-03-03 20:11:09 +01:00
m4 add m4/ subdir 2009-02-26 14:11:43 +01:00
rules udevd: switch watch symlinks to devpath 2009-03-05 00:35:58 +01:00
test test: add test for empty and non-existent ATTR 2009-02-05 12:40:15 +01:00
udev udevadm: settle - synchronise with the udev daemon 2009-03-12 10:02:37 +00:00
.gitignore gitignore: move *.8 to subdirs 2008-09-29 05:03:43 +02:00
autogen.sh autogen.sh: add more warnings 2009-02-05 19:01:27 +01:00
ChangeLog release 139 2009-02-26 23:28:20 +01:00
configure.ac libudev: ctrl - return error after sending ctrl message 2009-03-11 13:32:39 +01:00
COPYING update source file headers 2006-08-28 00:29:11 +02:00
INSTALL update INSTALL 2008-08-28 22:58:03 +02:00
Makefile.am add m4/ subdir 2009-02-26 14:11:43 +01:00
Makefile.am.inc libudev: get rid of selinux 2008-10-02 18:48:40 +02:00
NEWS version bump 2009-03-03 00:55:42 +01:00
README update NEWS 2009-01-04 17:38:33 +01:00
TODO update TODO 2009-03-07 02:35:18 +01:00
udev.conf udevadm: control - use getopt_long() 2008-09-03 21:56:47 +02:00

udev - userspace device management

For more information see the files in the docs/ directory.

Important Note:
  Integrating udev in the system has complex dependencies and differs from distro
  to distro. All major distros depend on udev these days and the system may not
  work without a properly installed version. The upstream udev project does not
  recommend to replace a distro's udev installation with the upstream version.

Requirements:
  - Version 2.6.22 of the Linux kernel for reliable operation of this release of
    udev. The kernel must not use the CONFIG_SYSFS_DEPRECATED* option.

  - The kernel must have sysfs, unix domain sockets and networking enabled.
    Unix domain sockets (CONFIG_UNIX) as a loadable kernel module is not
    supported.

  - The proc filesystem must be mounted on /proc/, the sysfs filesystem must
    be mounted at /sys/. No other locations are supported by udev.

  - The system must have the following group names resolvable at udev startup:
      disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, kmem.
    Especially in LDAP setups, it is required, that getgrnam() is able to resolve
    these group names with only the rootfs mounted, and while no network is
    available.

Operation:
  Udev creates and removes device nodes in /dev/, based on events the kernel
  sends out on device discovery or removal.

  - Very early in the boot process, the /dev/ directory should get a 'tmpfs'
    filesystem mounted, which is populated from scratch by udev. Created nodes
    or changed permissions will not survive a reboot, which is intentional.

  - The content of /lib/udev/devices/ directory which contains the nodes,
    symlinks and directories, which are always expected to be in /dev, should
    be copied over to the tmpfs mounted /dev, to provide the required nodes
    to initialize udev and continue booting.

  - The old hotplug helper /sbin/hotplug should be disabled on bootup, before
    actions like loading kernel modules are taken, which may cause a lot of
    events.

  - The udevd daemon must be started on bootup to receive netlink uevents
    from the kernel driver core.

  - All kernel events are matched against a set of specified rules in
    /lib/udev/rules.d/ which make it possible to hook into the event
    processing to load required kernel modules and setup devices. For all
    devices the kernel exports a major/minor number, udev will create a
    device node with the default kernel name, or the one specified by a
    matching udev rule.

Please direct any comment/question/concern to the linux-hotplug mailing list at:
  linux-hotplug@vger.kernel.org