https://blog.yusukeikoma.com/tags/linux/Recent content in linux on Ikoma's BlogHugoen-usSat, 03 Oct 2026 18:08:27 +0900https://blog.yusukeikoma.com/posts/replace-running-daemon-without-downtime/Sat, 03 Oct 2026 18:08:27 +0900https://blog.yusukeikoma.com/posts/replace-running-daemon-without-downtime/A runbook for replacing a running daemon without dropping requests, tested on a small TCP server under real systemd. A plain restart refused or reset connections every time; starting the new version beside the old one avoided refusals but still reset a connection in some runs, and a kernel setting made those resets go away; socket activation produced no errors. Also covered: draining in-flight work against TimeoutStopSec, and a release swap that checks the new version and rolls back.https://blog.yusukeikoma.com/posts/systemd-killmode-self-update/Mon, 28 Sep 2026 09:48:47 +0900https://blog.yusukeikoma.com/posts/systemd-killmode-self-update/A daemon that updates itself by starting a helper and restarting its own unit has a trap: the helper starts inside the service’s cgroup, and systemd kills everything in that cgroup on restart. In a throwaway systemd, a setsid’d helper vanished before its last log line, KillMode=process kept it alive but leaked it into the next instance, and systemd-run gave it its own unit. This post shows the evidence, the fix, and what I did not test.https://blog.yusukeikoma.com/posts/two-small-writes-nagle-delayed-ack/Tue, 01 Sep 2026 20:56:28 +0900https://blog.yusukeikoma.com/posts/two-small-writes-nagle-delayed-ack/Send a header and a body as two small write() calls, then wait for a reply, and every round trip can stall for about 40 ms on an idle machine with an idle network. Neither Nagle’s algorithm nor delayed ACK is a bug; together they deadlock until a timer fires. This post predicts the stall, reproduces it with a 50-line script, reads it off a packet trace, and compares the four ways out.https://blog.yusukeikoma.com/posts/level-triggered-invalidation/Tue, 25 Aug 2026 17:33:01 +0900https://blog.yusukeikoma.com/posts/level-triggered-invalidation/epoll’s edge-triggered mode stalls after a partial read; level-triggered mode cannot. The same distinction decides whether a realtime client survives lost, duplicated and reordered events. A reproducible epoll experiment, a five-way simulation on a lossy channel, and a 40-line invalidator that handles the race everyone misses: an event that arrives while the refetch is running.https://blog.yusukeikoma.com/posts/tcp-half-open-connection-detection/Sun, 23 Aug 2026 15:09:06 +0900https://blog.yusukeikoma.com/posts/tcp-half-open-connection-detection/TCP never tells you that the peer is gone. A reader blocks forever, a writer keeps retransmitting for a quarter of an hour, and SO_KEEPALIVE does nothing while data is unacknowledged. This post builds a one-file Linux lab with no root, climbs a ladder of mechanisms (nothing, keepalive, TCP_USER_TIMEOUT, an application heartbeat), measures how long each takes to notice, and shows where NAT and the RFCs fit in.