[PATCH v5 3/3] cgroups: make procs file writable

[Li Zefan]

David Rientjes wrote:
> On Mon, 27 Dec 2010, Ben Blum wrote:
> 
>>> I'm not sure what the benefit of defining it as a macro would be.  You're 
>>> defining these statically allocated nodemasks so they have file scope, I 
>>> hope (so they can be shared amongst the users who synchronize on 
>>> cgroup_lock() already).
>> In the attach() case, yes, but in other cases I was thinking they could
>> be put on the stack if CONFIG_NODES_SHIFT < 8, and static but still
>> per-function otherwise. Or should all the functions share the same
>> global nodemask?
>>
> 
> I think it would be appropriate to use a shared nodemask with file scope 
> whenever you have cgroup_lock() to avoid the unnecessary kmalloc() even 
> with GFP_KERNEL.  Cpusets are traditionally used on very large machines in 
> the first place, so there is a higher likelihood that 
> CONFIG_NODES_SHIFT > 8 whenever CONFIG_CPUSETS is enabled.
> 
> All users of NODEMASK_ALLOC() should be protected by cgroup_lock() other 
> than cpuset_sprintf_memlist(), right?  That should be the only remaining 
> user of NODEMASK_ALLOC() and works well since it can return -ENOMEM.
> 

Changing cpuset->mems_allowed is protected by both cgroup_mutex and
cpuset-specific lock (callback_mutex), so you can read it under either
lock, so NODEMASK_ALLOC() is not needed. See cpuset_sprintf_cpulist().