Re: [LKP] dd2283f260 [ 97.263072] WARNING:at_kernel/locking/lockdep.c:#lock_downgrade

[Yang Shi]

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.

> I'll look into this more when I get some free cycles.

I'm still traveling, will definitely look into it further once I'm back.

Thanks,
Yang

> It's better to disable the optimization for now (by ignoring 'downgrade'
> in __do_munmap()). Before it hits release.