The Value Of Spinlocks

Jeremiah Peschka walks us through the concepts behind spinlocks:

Here’s our fist stab at a spinlock, badly written in C:

// lock_value is an integer that's shared between multiple threads
while (lock_value != 0) {
  // spin
}

lock_value = 1;

do_something();

lock_value = 0;

The general idea here is correct – we have some lock_value and we only want to allow processes into the critical section of code if the another process doesn’t “hold the lock.” Holding the lock means that lock_value is set to a non-zero value.

There are few things that make this code bad. These things, coincidentally, are part of what make concurrent programming difficult.

Spinlocks are a critical part of maintaining internal consistency, but they can also accidentally ruin performance on your system if done wrong.  Read the whole thing.