linux-kernel - Re: [LKP] dd2283f260 [ 97.263072] WARNING:at_kernel/locking/lockdep.c:#lock

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Date:   Wed, 21 Nov 2018 08:35:28 +0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     kernel test robot <rong.a.chen@...el.com>,
        Waiman Long <longman@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org, LKP <lkp@...org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [LKP] dd2283f260 [ 97.263072]
 WARNING:at_kernel/locking/lockdep.c:#lock_downgrade



On 11/20/18 9:42 PM, Kirill A. Shutemov wrote:
> On Tue, Nov 20, 2018 at 08:10:51PM +0800, Yang Shi wrote:
>>
>> On 11/20/18 4:57 PM, Kirill A. Shutemov wrote:
>>> On Fri, Nov 16, 2018 at 08:56:04AM -0800, Yang Shi wrote:
>>>>> a8dda165ec  vfree: add debug might_sleep()
>>>>> dd2283f260  mm: mmap: zap pages with read mmap_sem in munmap
>>>>> 5929a1f0ff  Merge tag 'riscv-for-linus-4.20-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/palmer/riscv-linux
>>>>> 0bc80e3cb0  Add linux-next specific files for 20181114
>>>>> +-----------------------------------------------------+------------+------------+------------+---------------+
>>>>> |                                                     | a8dda165ec | dd2283f260 | 5929a1f0ff | next-20181114 |
>>>>> +-----------------------------------------------------+------------+------------+------------+---------------+
>>>>> | boot_successes                                      | 314        | 178        | 190        | 168           |
>>>>> | boot_failures                                       | 393        | 27         | 21         | 40            |
>>>>> | WARNING:held_lock_freed                             | 383        | 23         | 17         | 39            |
>>>>> | is_freeing_memory#-#,with_a_lock_still_held_there   | 383        | 23         | 17         | 39            |
>>>>> | BUG:unable_to_handle_kernel                         | 5          | 2          | 4          | 1             |
>>>>> | Oops:#[##]                                          | 9          | 3          | 4          | 1             |
>>>>> | EIP:debug_check_no_locks_freed                      | 9          | 3          | 4          | 1             |
>>>>> | Kernel_panic-not_syncing:Fatal_exception            | 9          | 3          | 4          | 1             |
>>>>> | Mem-Info                                            | 4          | 1          |            |               |
>>>>> | invoked_oom-killer:gfp_mask=0x                      | 1          | 1          |            |               |
>>>>> | WARNING:at_kernel/locking/lockdep.c:#lock_downgrade | 0          | 6          | 4          | 7             |
>>>>> | EIP:lock_downgrade                                  | 0          | 6          | 4          | 7             |
>>>>> +-----------------------------------------------------+------------+------------+------------+---------------+
>>>>>
>>>>> [   96.288009] random: get_random_u32 called from arch_rnd+0x3c/0x70 with crng_init=0
>>>>> [   96.359626] input_id (331) used greatest stack depth: 6360 bytes left
>>>>> [   96.749228] grep (358) used greatest stack depth: 6336 bytes left
>>>>> [   96.921470] network.sh (341) used greatest stack depth: 6212 bytes left
>>>>> [   97.262340]
>>>>> [   97.262587] =========================
>>>>> [   97.263072] WARNING: held lock freed!
>>>>> [   97.263536] 4.19.0-06969-gdd2283f #1 Not tainted
>>>>> [   97.264110] -------------------------
>>>>> [   97.264575] udevd/198 is freeing memory 9c16c930-9c16c99b, with a lock still held there!
>>>>> [   97.265542] (ptrval) (&anon_vma->rwsem){....}, at: unlink_anon_vmas+0x14e/0x420
>>>>> [   97.266450] 1 lock held by udevd/198:
>>>>> [   97.266924]  #0: (ptrval) (&mm->mmap_sem){....}, at: __do_munmap+0x531/0x730
>>>> I have not figured out what this is caused by. But, the below warning looks
>>>> more confusing. This might be caused by the below one.
>>> I *think* we need to understand more about what detached VMAs mean for
>>> rmap. The anon_vma for these VMAs still reachable for the rmap and
>>> therefore VMA too. I don't quite grasp what is implications of this, but
>>> it doesn't look good.
>> I'm supposed before accessing anon_vma, VMA need to be found by find_vma()
>> first, right? But, finding VMA need hold mmap_sem, once detach VMAs is
>> called, others should not be able to find the VMAs anymore. So, the anon_vma
>> should not be reachable except the munmap caller.
> No. anon_vma can be reached from page->mapping. The page can be reached
> during physcal memory scan or if the page is shared (across fork()). None
> of these accesses require mmap_sem.

If they don't require mmap_sem at all, this problem should be valid 
regardless of the optimization. We just downgraded write mmap_sem to 
read, but still hold it.

>