A kernel bug froze my machine: Debugging an async-profiler deadlock

Summary

QuestDB is the open-source time-series database for demanding workloads—from trading floors to mission control. It delivers ultra-low latency, high ingestion throughput, and a multi-tier storage engine. Native support for Parquet and SQL keeps your data portable, AI-ready—no vendor lock-in. I've been a Linux user since the late 90s, starting with Slackware on an underpowered AMD K6. Over the years I've hit plenty of bugs, but the last decade has been remarkably stable - until a kernel bug started freezing my machine whenever I used async-profiler. I'm not a kernel developer, but I found myself poking around kernel source code to understand the problem better and figure out what was going on under the hood. I was about to start an investigation of latency spikes in QuestDB reported by a user. To do that, I wanted to use the async-profiler to capture CPU heatmaps. Async-profiler heatmap example However, when I tried to attach the profiler, my machine froze completely. It did not respond to any keys, it was impossible to switch to a terminal, it did not respond to SSH. The only way to recover was to hard reboot it. I tried to start QuestDB with the profiler already configured to start at launch - the same result, a frozen machine almost immediately after the launch. I thought that was weird, this had not happened to me in years. It was already late in the evening, I felt tired anyway so I decided to call it a day. There was a tiny chance I was hallucinating and the problem would go away by itself overnight. A drowning man will clutch at a straw after all. The next day, I tried to attach the profiler again - same result, frozen machine. Async-profiler integration in QuestDB is a relatively new feature, so I thought there might be a bug in the integration code, perhaps a regression in the recent QuestDB release. So I built an older QuestDB version: The same result, frozen machine. This was puzzling - I positively knew this worked before. How do I know? Because I worked o...