https://cs.lozic.me/areas/operating-systems/Recent content in Operating-Systems on CS TheoremsHugoen-usFri, 24 Dec 2027 12:00:00 +0100https://cs.lozic.me/posts/t052-mutual-exclusion-without-hardware-support/Fri, 24 Dec 2027 12:00:00 +0100https://cs.lozic.me/posts/t052-mutual-exclusion-without-hardware-support/<h2 id="symptom">Symptom</h2> <p>Two threads increment a shared counter a million times each. The final value is 1,374,522.</p> <p>You know why: the increment is a read, an add, and a write, and the interleavings lose updates. The fix is a lock. But a lock is provided by your runtime, which gets it from the OS, which gets it from a hardware instruction — <code>lock xchg</code>, <code>cmpxchg</code>, <code>ldrex</code>/<code>strex</code>.</p>https://cs.lozic.me/posts/t051-the-coffman-conditions/Fri, 29 Jan 2027 12:00:00 +0100https://cs.lozic.me/posts/t051-the-coffman-conditions/<h2 id="symptom">Symptom</h2> <p>Production is wedged. Not slow — wedged. CPU is at zero, no errors are being logged, and two threads are sitting in a lock wait with no timeout. The stack traces show thread A holding the account lock and waiting for the ledger lock, thread B holding the ledger lock and waiting for the account lock. Somebody restarts the service and it goes away.</p>