Hi, I think there may be a plausible issue here. If not, maybe I'll get an education :)
cpu_callin_map is used during secondary CPU bootstrap to notify the waiting CPU that the new CPU is coming up. __cpu_up clears cpu_callin_map[cpu] and then polls the same location, waiting for start_secondary to set it to 1. But I'm wondering how safe the current implementation is -- start_secondary doesn't have an explicit sync following cpu_callin_map[cpu] = 1, and __cpu_up has no synchronization instructions in its polling loop, so how can we be sure that the waiting cpu will see the update to that location in time? Compare with the prom_hold_cpus/__secondary_hold_acknowledge code, which is doing a very similar task, but it has the mb and sync (in head_64.S at least) that seem to be missing from the case above. Since we're not buried in "Processor X is stuck" bug reports, I must be missing something, or there's some incidental factor that makes it okay in practice... Relevant code from arch/powerpc/kernel/smp.c: static volatile unsigned int cpu_callin_map[NR_CPUS]; .... int __cpuinit __cpu_up(unsigned int cpu) { int c; secondary_ti = current_set[cpu]; if (!cpu_enable(cpu)) return 0; if (smp_ops == NULL || (smp_ops->cpu_bootable && !smp_ops->cpu_bootable(cpu))) return -EINVAL; /* Make sure callin-map entry is 0 (can be leftover a CPU * hotplug */ cpu_callin_map[cpu] = 0; /* The information for processor bringup must * be written out to main store before we release * the processor. */ smp_mb(); /* wake up cpus */ DBG("smp: kicking cpu %d\n", cpu); smp_ops->kick_cpu(cpu); /* * wait to see if the cpu made a callin (is actually up). * use this value that I found through experimentation. * -- Cort */ if (system_state < SYSTEM_RUNNING) for (c = 50000; c && !cpu_callin_map[cpu]; c--) udelay(100); #ifdef CONFIG_HOTPLUG_CPU else /* * CPUs can take much longer to come up in the * hotplug case. Wait five seconds. */ for (c = 25; c && !cpu_callin_map[cpu]; c--) { msleep(200); } #endif if (!cpu_callin_map[cpu]) { printk("Processor %u is stuck.\n", cpu); return -ENOENT; } printk("Processor %u found.\n", cpu); if (smp_ops->give_timebase) smp_ops->give_timebase(); /* Wait until cpu puts itself in the online map */ while (!cpu_online(cpu)) cpu_relax(); return 0; } .... int __devinit start_secondary(void *unused) { unsigned int cpu = smp_processor_id(); struct device_node *l2_cache; int i, base; atomic_inc(&init_mm.mm_count); current->active_mm = &init_mm; smp_store_cpu_info(cpu); set_dec(tb_ticks_per_jiffy); preempt_disable(); cpu_callin_map[cpu] = 1; smp_ops->setup_cpu(cpu); if (smp_ops->take_timebase) smp_ops->take_timebase(); .... _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@ozlabs.org https://ozlabs.org/mailman/listinfo/linuxppc-dev