linux-kernel - Re: WARNING: locking bug in lock

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Date:   Thu, 13 Dec 2018 14:46:13 -0500
From:   Waiman Long <longman@...hat.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>,
        syzbot <syzbot+53383ae265fb161ef488@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux@...inikbrodowski.net, mhocko@...e.com, rientjes@...gle.com,
        syzkaller-bugs@...glegroups.com, vbabka@...e.cz,
        peterz@...radead.org, "mingo@...hat.com" <mingo@...hat.com>,
        boqun.feng@...il.com
Subject: Re: WARNING: locking bug in lock_downgrade

On 12/12/2018 08:14 PM, Yang Shi wrote:
> Cc'ed Peter, Ingo and Waiman.
>
>
> It took me a few days to look into this warning, but I got lost in
> lockdep code.
>
>
> The problem is the commit dd2283f2605e ("mm: mmap: zap pages with read
> mmap_sem in munmap") does an optimization for munmap by downgrading
> write mmap_sem to read before zapping pages. But, lockdep reports
> downgrading a read lock.
>
>
> I'm pretty sure mmap_sem is held as write before downgrade_write() is
> called in the patch. And, there are 4 places which may downgrade a
> mmap_sem:
>
>     - munmap
>
>     - mremap
>
>     - brk
>
>     - clear_refs_write (fs/proc/task_mmu.c)
>
>
> The first three come from my patches, and they just do:
> down_write_killable() -> .. -> downgrade_write().
>
> But the last one is a little bit more complicated, it does down_read()
> ->.. -> up_read() ->.. -> down_write_killable() ->.. ->
> downgrade_write().
>
> And, the last one may be called from any process to touch the other
> processes' mmap_sem.
>
>
> By looking into lockdep code, I'm not sure if lockdep may get confused
> by such sequence or not?
>
>
> Any hint is appreciated.
>
>
> Regards,
>
> Yang 

The warning was printed because hlock->read was set when doing the
downgrade_write(). So it is either downgrade_write() was called a second
time or a read lock was held originally. It is hard to tell what is the
root cause without a reproducer.

Cheers,
Longman