Bug: ready queue updates are not atomic

[pbalduino]

Summary

ready_queue_push updates the scheduler ready queues without a guaranteed atomic/scheduler-protected context on every caller path. In particular, proc_execute can enqueue a newly created process while interrupts are still enabled.

A timer interrupt can enter proc_switch while the ready queue list is mid-update, allowing the scheduler to traverse partially linked nodes and corrupt the queue.

Current Notes

Tracked locally in scheduler_issues.md as "Ready Queue Updates Are Not Atomic".

Relevant code:

• src/kernel/proc/proc.c: ready_queue_push
• src/kernel/proc/proc.c: proc_execute enqueue path
• src/kernel/proc/proc.c: proc_mark_ready guarded path for comparison

Expected Behavior

All ready queue mutations should happen under one clear scheduler invariant: either interrupts are disabled around the mutation, or a scheduler/runqueue lock protects the list consistently.

Proposed Fix

Mirror the guarded path used by proc_mark_ready, or introduce a scheduler/runqueue lock and route all ready queue insertion/removal through that contract.

Dependencies

• Issue Implement preemptive userland scheduler #34: Preemptive scheduler (CLOSED)
• Issue Implement spinlock synchronization primitive #38: Spinlock implementation (CLOSED)

Blocks

• Issue Extend scheduler for SMP with per-CPU runqueues #83: Extend scheduler for SMP with per-CPU runqueues

Related Issues

• Issue Enable SMP (Symmetric Multiprocessing) support #42: Enable SMP support
• Issue Bug: process PID table is not populated #426: Process PID table is not populated
• Issue Bug: scheduler process cleanup leaks resources #427: Scheduler process cleanup leaks resources

Validation

• Add or update scheduler tests/stress coverage for process creation and ready queue transitions where practical.
• Boot smoke test with repeated fork/exec/wait workloads.
• Confirm make test still passes.