Hi Tejun,

On 2018/2/22 下午11:18, Tejun Heo wrote:
> Hello,
> On Thu, Feb 22, 2018 at 02:14:34PM +0800, Joseph Qi wrote:
>> I still don't get how css_tryget can work here.
>> The race happens when:
>> 1) writeback kworker has found the blkg with rcu;
>> 2) blkcg is during offlining and blkg_destroy() has already been called.
>> Then, writeback kworker will take queue lock and access the blkg with
>> refcount 0.
> Yeah, then tryget would fail and it should go through the root.
In this race, the refcount of blkg becomes zero and is destroyed.
However css may still have refcount, and css_tryget can return success
before other callers put the refcount.
So I don't get how css_tryget can fix this race? Or I wonder if we can
add another function blkg_tryget?


> Thanks.

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