[$] The kernel "closure" API The data structure known as a "closure" first found its way into the mainline kernel with the addition of https://bcache.evilpiepirate.org/ in the 3.10 development cycle. With the advent of https://bcachefs.org/ in 6.7, though, it acquired a second user and was moved to the kernel's lib directory, making it available to other kernel users as well. The documentation of closures in the source is better than that of many things in the kernel, but there is still room for a gentler introduction. https://lwn.net/Articles/957187/