https://blog.hoshiyomi-dev.work/tags/concurrency/Recent content in Concurrency on BlogHugoen-usSun, 11 Oct 2026 10:54:05 +0000https://blog.hoshiyomi-dev.work/post/kcalc-self-deadlock/Sat, 22 Aug 2026 02:45:35 +0800https://blog.hoshiyomi-dev.work/post/kcalc-self-deadlock/<h2 id="context">Context</h2> <p><a href="https://github.com/xDDoubleTea/kcalc">kcalc</a> is a calculator running directly in kernelspace. Achieved by loading as a Linux kernel module, and by exposing a character device <code>/dev/kcalc_chardev</code> to the userspace, supporting <code>read, write</code> operations.</p> <ul> <li>A process <code>write</code> the expression they want to evaluate to <code>/dev/kcalc_chardev</code>.</li> <li>That process then <code>read</code> from <code>/dev/kcalc_chardev</code>.</li> <li>The character device is locked whenever a process is writing to/reading from it via <code>mutex_lock();</code>.</li> <li>The kernel is tested using <code>pytest</code>, calls <code>tee</code> and <code>cat</code> for <code>write</code> and <code>read</code> operations respectively</li> </ul> <h2 id="what-happened">What Happened</h2> <p>When running the <code>pytest</code> suite, the process hangs indefinitely, no cases is passed or failed. It is also unkillable. Opening <code>btop</code> reveals that the <code>cat</code> process is there hanging, and causing the <code>pytest</code> which calls <code>cat</code> as a subprocess hangs. The <code>cat</code> process is unkillable even with <code>SIGKILL</code>.</p>