id	summary	reporter	owner	description	type	status	priority	milestone	component	version	severity	resolution	keywords	cc
864	Races with device creation when udev is used with a modular kernel	alexander@…	lfs-book@…	"It is a known fact that device creation with udev in response to hotplug enevts
(those generated after modules load) is asynchronous. So the device creation
happens a bit after the modprobe command finishes.

A result is that one can no longer assume that when a module finished loaded
(i.e., the modprobe command returned) it is usable. This assumption fails
because the device might have not been created yet (i.e., race of device
creation vs its use).

A known victim is the ALSA initscript in BLFS if the soundcard module (e.g.,
snd-fm801) is loaded by hotplug. There may be others.

An upstream ""wisdom"" is to abandon the fundamental concept of executing
initscripts synchronously and sequentially. One has to put handlers into
/etc/dev.d instead. However, this ""solution"" doesn't work.

First, these handlers are not called by udevstart, so people who compiled
snd-fm801 as a non-module need an initscript, not a /etc/dev.d handler, to
restore volumes. We have to support two cases instead of one.

Then, there is no documented way (see undocumented way below) to _wait_ (as
opposed to putting a handler) for completion of the process of creation of all
devices that should be created as a result of loading a module. The upstream
""wisdom"" is: you should not need to wait, you should always put a handler -- but
this works only when a device name is known, unlike the case when the module is
probed by coldplug scripts.

All of the above means that there is _no_ documented way to safely use udev + 
modules. So we should either drop udev, or completely drop support for modular
kernels, or accept a hack below.

The hack is based on two undocumented facts:

1) Creation of sysfs entries is synchronous, i.e. all of them are guaranteed to
exist when modprobe finishes
2) udevstart is synchronous, i.e. all device nodes corresponding to existing
sysfs entries are guaranteed to exist when udevstart finishes

So I propose to add a second call to udevstart to the end of the start section
of the hotplug initscript, to create all device nodes corresponding to modules
loaded by hotplug. Of course it should be surrounded by the check if
/sbin/udevstart is executable, and a pointer to this bug must be in the comments
above this hack.

This fixes a problem with ALSA initscript and in fact all races caused by
modprobe commands in the hotplug initscript.

It doesn't fix the possible problem with other applications who probe the module
themselves (DRI/Xorg? can't check) and expect the devices to be immediately
usable. This problem is unfixable and must be documented in the LFS book.

Oh, there is! Create a wrapper around modprobe that calls udevstart after the
real modprobe :)"	defect	closed	normal		Book	SVN	critical	fixed