Chapter 3. Locking in the Linux Kernel

Table of Contents

Two Main Types of Kernel Locks: Spinlocks and Mutexes
Locks and Uniprocessor Kernels
Locking Only In User Context
Locking Between User Context and Softirqs
Locking Between User Context and Tasklets
Locking Between User Context and Timers
Locking Between Tasklets/Timers
The Same Tasklet/Timer
Different Tasklets/Timers
Locking Between Softirqs
The Same Softirq
Different Softirqs

If I could give you one piece of advice: never sleep with anyone crazier than yourself. But if I had to give you advice on locking: keep it simple.

Be reluctant to introduce new locks.

Strangely enough, this last one is the exact reverse of my advice when you have slept with someone crazier than yourself. And you should think about getting a big dog.

Two Main Types of Kernel Locks: Spinlocks and Mutexes

There are two main types of kernel locks. The fundamental type is the spinlock (include/asm/spinlock.h), which is a very simple single-holder lock: if you can't get the spinlock, you keep trying (spinning) until you can. Spinlocks are very small and fast, and can be used anywhere.

The second type is a mutex (include/linux/mutex.h): it is like a spinlock, but you may block holding a mutex. If you can't lock a mutex, your task will suspend itself, and be woken up when the mutex is released. This means the CPU can do something else while you are waiting. There are many cases when you simply can't sleep (see Chapter 10, What Functions Are Safe To Call From Interrupts?), and so have to use a spinlock instead.

Neither type of lock is recursive: see the section called “Deadlock: Simple and Advanced”.