Function critical_section::acquire

Acquire a critical section in the current thread.

This function is extremely low level. Strongly prefer using with instead.

Nesting critical sections is allowed. The inner critical sections are mostly no-ops since they’re already protected by the outer one.

Safety

• Each acquire call must be paired with exactly one release call in the same thread.
• acquire returns a “restore token” u8 that you must pass to the corresponding release call, and treat opaquely otherwise.
• acquire/release pairs must be “properly nested”, ie it’s not OK to do a=acquire(); b=acquire(); release(a); release(b);.
• It is UB to call release if the critical section is not acquired in the current thread.
• It is UB to call release with a restore token that does not come from the corresponding acquire call.