CODEWITHSUNDEEP
linuxlinux-kerneloperating-systemlinux-device-driver
Bandicoot
Bandicoot

Reputation: 3949

wait queues vs semaphores in linux

Why do we use wait queues in the linux kernel rather than use semaphores for synchronization ? What is the difference between using wait queues vs semaphores for synchronization ?

Upvotes: 3

Views: 3865

Answers (1)

Anirudh Ramanathan
Anirudh Ramanathan

Reputation: 46728

A semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access by multiple processes to a common resource in a parallel programming environment. (Wikipedia)

Now, a semaphore is more of a concept, rather than a specific implementation.

The linux semaphore data structure implementation uses a wait-queue. Without a wait queue, you wouldn't know which process demanded the resource first, which could lead to very large wait times for some. The wait-queue ensures fairness, and abates the resource starvation problem. 

struct semaphore {
    int count; //+ve or -ve indicates resource free/busy state
    int waking; //number of waiting processes
    int lock ;  /* to make waking testing atomic */
    struct wait_queue *wait; //queued, to prevent starvation, ensure fairness
};

Reference

Upvotes: 4

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