Re: [BUG??] Deadlock between kswapd and sys_inotify_add_watch(lockdep report)

[MinChan Kim <minchan.kim@gmail.com>]

On Mon, Feb 2, 2009 at 10:09 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, 2009-02-02 at 20:56 +0900, MinChan Kim wrote:
>> Thanks for kind explanation. :)
>> Unfortunately, I still have a question. :(
>
> No problem :-)
>
>> > > I think if reclaim context which have GFP_FS already have lock A and then
>> > > do pageout, if writepage need the lock A, we have to catch such a case.
>> > > I thought Nick's patch's goal catchs such a case.
>> >
>> > Correct, it exactly does that.
>>
>> But, I think such a case can be caught by lockdep of recursive detection
>> which is existed long time ago by making you.
>
> (Ingo wrote that code)
>
>> what's difference Nick's patch and recursive lockdep ?
>
> Very good question indeed. Every time I started to write an answer I
> realize its wrong.
>
> The below is half the answer:
>
> /*
>  * Check whether we are holding such a class already.
>  *
>  * (Note that this has to be done separately, because the graph cannot
>  * detect such classes of deadlocks.)
>  *
>  * Returns: 0 on deadlock detected, 1 on OK, 2 on recursive read
>  */
> static int
> check_deadlock(struct task_struct *curr, struct held_lock *next,
>               struct lockdep_map *next_instance, int read)
>
> So in order for the reclaim report to trigger we have to actually hit
> that code path that has the recursion in it. The reclaim context
> annotation by Nick ensures we detect such cases without having to do
> that.

In my case and Nick's patch's example hit code path that has the
recursion in it.
then reported it.

Do I miss something ?

> The second half, to which I cannot seem to get a decent answer to atm,
> is why the recursion case isn't detected by the graph.
>
>
>
>



-- 
Kinds regards,
MinChan Kim

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