Non-monotonic output timeline in mixed M-mode/U-mode execution

[HeatCrab]

When M-mode and U-mode tasks execute concurrently, the log output exhibits a temporally regressive pattern. Iteration counts from M-mode tasks jump forward, then retroactively fill in earlier values. Meanwhile, U-mode iteration counts remain strictly monotonic.

The following snippet from privilege_switch, which is introduced in PR #32, test output illustrates this behavior:

[M-mode] iteration 16253
...
[M-mode] iteration 16260
[U-mode] iteration 497
[M-mode] iteration 16292    ← jumps forward
...
[M-mode] iteration 16253    ← fills in earlier values

Notably, the total message count remains correct—only the display order is affected. This suggests the underlying execution is sound, but the I/O path introduces ordering anomalies when mixing privilege modes.

This behavior makes it difficult to trace concurrent M-mode and U-mode interactions through logs, as the temporal sequence does not reflect actual execution order.

Related Issues:

• Output Interleaving due to Unprotected I/O Loops #64
• DoS vulnerability in sys_tputs through unbounded NOSCHED blocking #65