From: eLinux.org
Asynchronous function calls
In order to make the kernel boot faster, a set of patches was introduced by Arjan van de Ven in January 2009 to create infrastructure to allow doing some of the initialization steps asynchronously. The patches allow overlapping significant portions of the hardware delays in practice. Asynchronous function calls has been merged in mainline starting from 2.6.29. Starting from 2.6.30 the asynchronous function call infrastructure is enabled by default.
In order to not change device order and other similar observables, the patch does NOT do full parallel initialization.
Rather, it operates more in the way an out of order CPU does; the work may be done out of order and asynchronous, but the observable effects (instruction retiring for the CPU) are still done in the original sequence.
References
This work was described in the LWN.net article An asynchronous function call infrastructure
See http://lkml.org/lkml/2009/1/4/155 for the first patch in the series.
Work similar in spirit to this was done previously, but with smaller scope and apparently not mainlined.
Status
This work was mainlined in kernel version 2.6.30 and is now found in the source file kernel/async.c
