Big Kernel Lock

1.3. No BKL (Big Kernel Lock)
The Big Kernel Lock is a giant lock that was introduced in Linux 2.0, when Alan Cox introduced SMP support for first time. But it was just an step to achieve SMP scalability - only one process can run kernel code at the same time in Linux 2.0, long term the BKL must be replaced by fine-grained locking to allow multiple processes running kernel code in parallel. In this version, it is possible to compile a kernel completely free of BKL support. Note that this doesn't have performance impact: all the critical Linux codepaths have been BKL-free for a long time. It still was used in many non-performance critical places -ioctls, drivers, non-mainstream filesystems, etc-, which are the ones that are being cleaned up in this version. But the BKL is being replaced in these places with mutexes, which doesn't improve parallelism (these places are not performance critical anyway).

Linux 2 6 37 - Linux Kernel Newbies