Table of Contents
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.
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 semaphore
(include/asm/semaphore.h): it
can have more than one holder at any time (the number decided at
initialization time), although it is most commonly used as a
single-holder lock (a mutex). If you can't get a semaphore,
your task will put itself on the queue, and be woken up when the
semaphore is released. This means the CPU will do something
else while you are waiting, but there are many cases when you
simply can't sleep (see Chapter 9, 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”.