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Atomic context

This lint will error on calls that violate kernel locking rules.

Conceptually this lint tracks the kernel preempt_count statically. All functions have two properties, first is the adjustment to the preempt_count that happens after calling the function, and the second is the expectation of preempt_count at the entrance to the function. The lint will keep track of the preempt_count within the function call, and error if the expectation is not met.

For example, this code is invalid:

let guard = spinlock.lock();
sleep();

because acquiring a spinlock disables preemption, but sleep expects preemption to be enabled.

With Spinlock::lock being annotated with #[klint::preempt_count(adjust = 1)] and sleep being annotated with #[klint::preempt_count(expect = 0)], the lint will generate the following error:

error: this call expects the preemption count to be 0
 --> example.rs:2:1
  |
2 | sleep();
  | ^^^^^^^
  |
  = note: but the possible preemption count at this point is 1..