Re: possible deadlock in get_user_pages_unlocked

[Al Viro]

On Thu, Feb 01, 2018 at 04:58:00PM -0800, syzbot wrote:
> Hello,
> 
> syzbot hit the following crash on upstream commit
> 7109a04eae81c41ed529da9f3c48c3655ccea741 (Thu Feb 1 17:37:30 2018 +0000)
> Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/ide
> 
> So far this crash happened 2 times on upstream.
> C reproducer is attached.

Umm...  How reproducible that is?

> syzkaller reproducer is attached.
> Raw console output is attached.
> compiler: gcc (GCC) 7.1.1 20170620
> .config is attached.

Can't reproduce with gcc 5.4.1 (same .config, same C reproducer).

It looks like __get_user_pages_locked() returning with *locked zeroed,
but ->mmap_sem not dropped.  I don't see what could've lead to it and
attempts to reproduce had not succeeded so far...

How long does it normally take for lockdep splat to trigger?