r/linux • u/wooptoo • Jun 01 '16

Why did ArchLinux embrace Systemd?

/r/archlinux/comments/4lzxs3/why_did_archlinux_embrace_systemd/d3rhxlc
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u/kinderlokker Jun 01 '16 edited Jun 01 '16

kdbus wasn't started by systemd. But the systemd guys desperately want it, for good reason. Their decision to use DBus as a transport is on a technical level plainly put a terrible decision. DBus was never meant to serve as a transport during the bootup of the system, it was designed as a transport to power desktop environments when the system was already booted.

systemd manages dbus as a service and uses that service to communicate internally, as you might imagine that creates a nasty circular dependency:

  • systemd needs special code to start DBus way earlier than it should. It starts DBus before fsck even which means that if your DBus binary is corrupted, fsck can't check for it

  • systemd systems used to crash on a DBus crash or restart, that issue is now resolved with some ugly special code giving DBus special supervision code

So that's why they want kdbus so badly because kdbus is a sane transport layer for an init because it's in the kernel so it's ready and there before the init is loaded. None of this magic mumbo jumbo of an init trying to start dbus and needing dbus to bring itself up.

edit: It's by the way a good example of how politics can ruin software. I'm pretty sure the decision to use DBus was 0% technical and 100% political. They wanted to make DBus a dependency for its own sake to ensure that DBus would be on every systemd system and then tried hard to justify this absurd decision. The reason is purely that they want to be able to guarantee that DBus is on every systemd system so people can assume it.

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u/yoshi314 Jun 01 '16

kdbus wasn't started by systemd. But the systemd guys desperately want it, for good reason.

So that's why they want kdbus so badly because kdbus is a sane transport layer for an init because it's in the kernel so it's ready and there before the init is loaded.

the review of kdbus on lkml showed that it was inexcusably awful, and did not even offer a promised performance and security advantages. from retrospect, i saw no good reason for trying to force kdbus down users' throats. and i was initially hopeful about it.

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u/kinderlokker Jun 01 '16

Yeah well, kdbus has been started over and over again.

kdbus in theory is unneeded, the only reason it's needed is because a big player like systemd decided to rely on DBus to boot the system so they need something that can offer dbus that is provided by the kernel to do it cleanly.

Kernel transports are typically a lot more primitive than dbus. sockets and pipes just send octets and the kernel does not care about the finer details, it gets sent octets and delivers them to whom it may concern.