A TASKS_RCU grace period is detected whose quiescent states comprise a task undergoing a voluntary context switch, a task running in user mode, and a task running in idle-mode. A list of all runnable tasks is built. The runnable task list is scanned in one or more scan passes. Each scan pass through the runnable task list searches to identify tasks that have passed through a quiescent state by either performing a voluntary context switch, running in user mode, or running in idle-mode. If found, such quiescent state tasks are removed from the runnable task list. Searching performed during a scan pass includes identifying quiescent state tickless user mode tasks that have been running continuously in user mode on tickless CPUs that have not received a scheduling clock interrupt since commencement of the TASKS_RCU grace period. If the runnable task list is empty, the TASKS_RCU grace period is ended.