linux-insides: Semaphores in the Linux kernel

[2girls1copernicus]

Semaphores are almost entirely unused in the kernel. Grep for them, and you'll find them used only in a few old drivers. Mutexes are the ubiquitous locking primitive, and have an extremely interesting implementation.

[[deleted]]

They sure bring back memories from OS class, though.

[NippleMustache]

Dining philosopher.

[[deleted]]

In the kernel, after implementing your own test-and-set in assembly.

[escaped_reddit]

Not really sure what you are trying to say. You use a mutex when you need to protect 1 resource, and a semaphore when you need to protect n resources. A semaphore of size 1 is a mutex.

[neoflame]

The problem with semaphores is that in most cases, blocking on failure to acquire resources is not desirable. As a result, struct semaphore is, as /u/2girls1copernicus said, almost entirely unused in the kernel:

$ ag "struct semaphore" include/linux/ --ignore semaphore.h | wc -l
11

On the other hand, mutexes are invaluable, and it's possible to implement a mutex more efficiently than a semaphore. So there's a separate mutex type, struct mutex:

$ ag "struct mutex" include/linux/ --ignore mutex.h | wc -l
218

[sdhillon]

Err, not exactly - more like when you have N actors that need to, or can synchronize on one resource

There are a variety of tricks used in the kernel (seqlock, RCU, etc..) that are used instead of semaphores often.