Linux 5.10 Will Have A Fully Lock-Less Ring Buffer
Linus Torvalds has merged a set of printk() patches by John Ogness that make the kernel ring buffer (what you see if you type
dmesg) fully lock-less. This is a big improvement that allows messages to be stored and read without the need for temporary per-CPU buffers with no risk of deadlocks.
written by 林慧 (Wai Lin). published 2020-10-17 - last edited 2020-10-24
The ring buffer is a central part of the Linux kernel. Drivers, sub-systems and a wide range of functionality use
printk() to to share errors and general messages about what's going on. You can see its contents by typing
sudo dmesg in a terminal.
John Ogness gave a 57 minute long video titled "Why is printk() so complicated?" detailing the history of the Linux kernel's ring buffer since Linux 0.01 (Linux-0.01.tar.gz) was released in 1991 to 2019 at the 2019 Linux Plumbers Conference. He pointed out the problems with the long-used
printk() implementation and proposed a set of ideas for improving
printk() and the kernel's ring buffer functionality at that conference. Some of of his work has now been merged to the Linux git tree, ensuring that it will be a part of Linux 5.10 when it gets released in a few months' time.
"The big new thing is the fully lockless ringbuffer implementation, including the support for continuous lines. It will allow to store and read messages in any situation wihtout the risk of deadlocks and without the need of temporary per-CPU buffers.
The access is still serialized by logbuf_lock. It synchronizes few more operations, for example, temporary buffer for formatting the message, syslog and kmsg_dump operations. The lock removal is being discussed and should be ready for the next release."
The remaining use of
logbuf_lock will be addressed when Linux 5.10 is released and the Linux 5.11 merge window opens up around Christmas-time.