commit a2db672aa305a045404615e5222ba681bab6cf58
author Silas Boyd-Wickizer <sbw@mit.edu> Fri Aug 03 12:33:27 2012 -0700
committer Paul E. McKenney <paulmck@linux.vnet.ibm.com> Sun Sep 23 07:43:56 2012 -0700
tree 49ac9a38d87e42695f212b730751dfa78cf3a0d4
parent 5d18023294abc22984886bd7185344e0c2be0daf

Use get_online_cpus to avoid races involving CPU hotplug

If arch/x86/kernel/msr.c is a module, a CPU might offline or online
between the for_each_online_cpu(i) loop and the call to
register_hotcpu_notifier in msr_init or the call to
unregister_hotcpu_notifier in msr_exit. The potential races can lead
to leaks/duplicates, attempts to destroy non-existant devices, or
random pointer dereferences.

For example, in msr_init if:

        for_each_online_cpu(i) {
                err = msr_device_create(i);
                if (err != 0)
                        goto out_class;
        }
        <----- CPU offlines
        register_hotcpu_notifier(&msr_class_cpu_notifier);

and the CPU never onlines before msr_exit, then the module will never
call msr_device_destroy for the associated CPU.

This fix surrounds for_each_online_cpu and register_hotcpu_notifier or
unregister_hotcpu_notifier with get_online_cpus+put_online_cpus.

Tested on a VM.

Signed-off-by: Silas Boyd-Wickizer <sbw@mit.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>