The full form of BKL is Big Kernel Lock.

The big kernel lock (BKL) is an old serialization method that we are trying to get rid of, replacing it with more fine-grained locking, in particular mutex, spinlock and RCU, where appropriate.

The BKL is a recursive lock, meaning that you can take it from a thread that already holds it. This may sound convenient, but easily introduces all sorts of bugs. Another problem is that the BKL is automatically released when a thread sleeps. This avoids lock order problems with mutexes in some circumstances, but also creates more problems because it makes it really hard to track what code is executed under the lock.

Many changes have been made and gone upstream, but some areas remain that we need to take care of independently: