Re: [RFC/PATCH] No get_user/put_user while holding mmap_sem in do_pages_stat?

[Brice Goglin <Brice.Goglin@inria.fr>]

Peter Zijlstra wrote:
> On Sun, 2008-12-07 at 15:21 +0100, Brice Goglin wrote:
>   
>> Andrew Morton wrote:
>>     
>>> Was lockdep able to tell you about this in any way?
>>>   
>>>       
>> With CONFIG_PROVE_LOCKING (assuming that it's enough), it doesn't detect
>> the problem for real. It just says "possible recursive locking detected"
>> between do_page_fault and sys_move_pages.
>>     
>
> That is real - how much more real do you need a description of a
> recursive deadlock to be?
>   

Well, it's a recursive down_read. It could be ok if we had the guarantee
that nobody else would be doing down_write in the middle. lockdep only
complained about this recursive down_read when there was a down_write
actually causing the deadlock, but it didn't say anything about this
down_write in the log.

It would be great if lockdep could say "recursive read-lock is
deadlocking because this other guy (with its backtrace) took for write
in the middle". I needed sysrq-t to get this info.

Brice