Re: [PATCH] percpu_counter: Remove debug_object_free call twice

[Kohli, Gaurav]

Hi ,

Sorry for very late reminder, just wanted to know is below understanding 
of code is wrong?

Regards
Gaurav

On 4/17/2018 11:59 AM, Kohli, Gaurav wrote:
> On 4/17/2018 3:18 AM, Tejun Heo wrote:
>
> > On Fri, Apr 13, 2018 at 03:05:03PM +0530, Gaurav Kohli wrote:
> > > During percpu_counter destroy, debug_object_free is calling
> > > twice which may create race. So removing once instance of call
> > > from debug_percpu_counter_deactivate.
> > I don't quite follow. Can you please elaborate how it can be called
> > twice?
>
> Hi Tejun,
>
> In percpu_counter_fixup_free function, first call is 
> percpu_counter_destroy -> debug_percpu_counter_deactivate (this will set 
> inactive and free the debug object as well for percpu counter) -> 
> free_percpu (finally freeing the counters).
>
> Below is the code snippet:
>
>  ÂÂÂÂÂÂÂ case DEBUG_STATE_ACTIVE:
>
>  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ percpu_counter_destroy(fbc); -> first call
>
>  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ debug_object_free(fbc, &percpu_counter_debug_descr); -> 
> this will again call the same debug object free, if somehow counters 
> will reinitialize between these two calls.
>
>
> We are seeing one race condition issue where one object of db list is 
> corrupted and just before object of that corrupted node is 
> percpu_counter, Still i am not sure the reason of race as it is very
>
> difficult to reproduce. But i have found this during code review. Please 
> correct me, if i misunderstood this.
>
> > Thanks.

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