lkml.org 
[lkml]   [2018]   [Feb]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: possible deadlock in get_user_pages_unlocked
On Thu, Feb 01, 2018 at 09:35:02PM -0800, Eric Biggers wrote:

> Try starting up multiple instances of the program; that sometimes helps with
> these races that are hard to hit (since you may e.g. have a different number of
> CPUs than syzbot used). If I start up 4 instances I see the lockdep splat after
> around 2-5 seconds.

5 instances in parallel, 10 minutes into the run...

> This is on latest Linus tree (4bf772b1467). Also note the
> reproducer uses KVM, so if you're running it in a VM it will only work if you've
> enabled nested virtualization on the host (kvm_intel.nested=1).

cat /sys/module/kvm_amd/parameters/nested
1

on host

> Also it appears to go away if I revert ce53053ce378c21 ("kvm: switch
> get_user_page_nowait() to get_user_pages_unlocked()").

That simply prevents this reproducer hitting get_user_pages_unlocked()
instead of grab mmap_sem/get_user_pages/drop mmap_sem. I.e. does not
allow __get_user_pages_locked() to drop/regain ->mmap_sem.

The bug may be in the way we call get_user_pages_unlocked() in that
commit, but it might easily be a bug in __get_user_pages_locked()
exposed by that reproducer somehow.

\
 
 \ /
  Last update: 2018-02-02 06:46    [W:0.070 / U:0.160 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site