s6-linux-utils: the s6-devd program

From 1acdafbf731ac703fdb24916c4be6424405170b7 Mon Sep 17 00:00:00 2001 From: Laurent Bercot Date: Mon, 9 Mar 2015 17:34:30 +0000 Subject: - s6-devd doc update - cosmetic doc fixes --- doc/s6-devd.html | 44 +++++++++++++++++++++----------------------- 1 file changed, 21 insertions(+), 23 deletions(-) (limited to 'doc/s6-devd.html') diff --git a/doc/s6-devd.html b/doc/s6-devd.html index c67429a..60fee03 100644 --- a/doc/s6-devd.html +++ b/doc/s6-devd.html @@ -4,7 +4,7 @@ s6-linux-utils: the s6-devd program - + @@ -26,20 +26,19 @@ interface does.

Interface

-     s6-devd [ -q | -v ] [ -b kbufsz ] [ -t l:t:k ] prog...
+     s6-devd [ -q | -v ] [ -b kbufsz ] [ -l linevar ] [ -t l:t:k ] prog...

s6-devd binds to the netlink interface and listens for -hotplug events, as the udev program does.
For every event it receives, it spawns prog... with -the event variables added to the environment, just as if prog... -had been registered in /proc/sys/kernel/hotplug.
However, unlike the kernel, s6-devd spawns the prog... helpers -sequentially: it waits for an instance to finish before spawning another one.
s6-devd is a long-lived program; it exits 0 when it receives a -SIGTERM. If a helper program is alive at that time, s6-devd waits for it to die -before exiting.
s6-devd rewrites its command line into +s6-uevent-listener +s6-uevent-spawner prog..., +dispatching its options to the appropriate program; then it execs into +this command line.
The s6-uevent-listener +will listen to the netlink and pass the events sequentially to +s6-uevent-spawner, which will +spawn a prog... helper for every event.

Options

@@ -47,9 +46,13 @@ before exiting.

-q : be more quiet.
-v : be more verbose.
-b kbufsz : try and reserve a kernel buffer of +
-b kbufsz : try and reserve a kernel buffer of kbufsz bytes for the netlink queue. Too large a buffer wastes kernel memory; too small a buffer risks losing events. The default is 65536.
-l linevar : the short description of the +event, given by the kernel as the first string in the netlink message, +will be made available to prog under the environment variable +named linevar.
-t l:t:k : If l, t or k is specified, they specify timeouts; by default, they are infinite. If prog... is still alive after l milliseconds, s6-devd sends @@ -62,18 +65,13 @@ alive after k more milliseconds, s6-devd yells and exits 99.

s6-devd is a daemon; it should be run under a proper supervision system such -as s6. (That is why it does not -fork and logs to stderr.)

s6-uevent-listener

The prog... helper, on the other hand, should be very short-lived, even if you are not using the -t option to s6-devd. Since helpers are -spawned sequentially, slow helpers can make events queue up and fill the netlink -kernel buffer.
If you are using busybox and want a -minimal udev-style dynamic /dev -handling, /sbin/mdev is a suitable prog... helper.
The point of s6-devd is that it runs the helpers sequentially, so it solves -the race condition that appears when helpers are run via the hotplug interface. -When s6-devd is used, /proc/sys/kernel/hotplug should be empty.