Gautham R Shenoy wrote: > Hi Nathan, > > Hi Gautham- > > > > Gautham R Shenoy wrote: > > > Replace all lock_cpu_hotplug/unlock_cpu_hotplug from the kernel and use > > > get_online_cpus and put_online_cpus instead as it highlights > > > the refcount semantics in these operations. > > > > Something other than "get_online_cpus", please? lock_cpu_hotplug() > > protects cpu_present_map as well as cpu_online_map. For example, some > > of the powerpc code modified in this patch is made a bit less clear > > because it is manipulating cpu_present_map, not cpu_online_map. > > A quick look at the code, and I am wondering why is lock_cpu_hotplug() > used there in the first place. It doesn't look like we require any > protection against cpus coming up/ going down in the code below, > since the cpu-hotplug operation doesn't affect the cpu_present_map.
The locking is necessary. Changes to cpu_online_map and cpu_present_map must be serialized; otherwise you could end up trying to online a cpu as it is being removed (i.e. cleared from cpu_present_map). Online operations must check that a cpu is present before bringing it up (kernel/cpu.c): /* Requires cpu_add_remove_lock to be held */ static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen) { int ret, nr_calls = 0; void *hcpu = (void *)(long)cpu; unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0; if (cpu_online(cpu) || !cpu_present(cpu)) return -EINVAL; .... > Can't we use another mutex here instead of the cpu_hotplug mutex here ? I guess so, but I don't really see the need... - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/