Messages in this thread |  | | | From | Keith Owens <> | | Subject | Is stopmachine() preempt safe? | | Date | Sun, 27 Aug 2006 19:42:32 +1000 |
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I cannot convince myself that stopmachine() is preempt safe. What prevents this race with CONFIG_PREEMPT=y?
cpu 0 cpu 1 stop_machine() Process <n> reads a global resource do_stop() kernel_thread(stopmachine, 1) Process <n> is preempted stopmachine() runs on cpu 1 STOPMACHINE_PREPARE STOPMACHINE_DISABLE_IRQ do_stop() calls smdata->fn smdata->fn changes global data restart_machine() STOPMACHINE_EXIT stopmachine() exits Scheduler resumes process <n> The global resource is out of sync
The stopmachine() threads on the other cpus are set to MAX_RT_PRIO-1 so they will preempt any existing process. The yield() in stopmachine() only guarantees that these kernel threads get onto the other cpus, it does not guarantee that all running tasks will proceed to a yield themselves before stopmachine runs. IOW, what guarantees that the scheduler will only run stopmachine() on the target cpus when those cpus are completely idle with no locally cached global resources?
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