wikipedia Giant lock
In operating systems, a giant lock, also known as a big-lock or kernel-lock, is a lock that may be used in the kernel to provide concurrency control required by symmetric multiprocessing (SMP) systems.
A giant lock is a solitary global lock that is held whenever a thread enters kernel space and released when the thread returns to user space; a system call is the archetypal example. In this model, threads in user space can run concurrently on any available processors or processor cores, but no more than one thread can run in kernel space; any other threads that try to enter kernel space are forced to wait. In other words, the giant lock eliminates all concurrency in kernel space.
By isolating the kernel from concurrency, many parts of the kernel no longer need to be modified to support SMP. However, as in giant-lock SMP systems only one processor can run the kernel code at a time, performance for applications spending significant amounts of time in the kernel is not much improved. Accordingly, the giant-lock approach is commonly seen as a preliminary means of bringing SMP support to an operating system, yielding benefits only in user space. Most modern operating systems use a fine-grained locking approach.
Linux
The Linux kernel had a big kernel lock (BKL) since the introduction of SMP, until Arnd Bergmann removed it in 2011 in kernel version 2.6.39, with the remaining uses of the big lock removed or replaced by finer-grained locking. Linux distributions at or above CentOS 7, Debian 7 (Wheezy) and Ubuntu 11.10 are therefore not using BKL.