Re: Revert commit 310ca162d77

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Mar 20, 2019 at 01:58:06PM +0100, Jan Kara wrote:
> Hello,
> 
> commit 310ca162d77 "block/loop: Use global lock for ioctl() operation." has
> been pushed to multiple stable trees. This patch is a part of larger series
> that overhauls the locking inside loopback device upstream and for 4.4,
> 4.9, and 4.14 stable trees only this patch from the series is applied. Our
> testing now has shown [1] that the patch alone makes present deadlocks
> inside loopback driver more likely (the openqa test in our infrastructure
> didn't hit the deadlock before whereas with the new kernel it hits it
> reliably every time). So I would suggest we revert 310ca162d77 from 4.4,
> 4.9, and 4.14 kernels.

Ugh, ok.

> Another option would be to backport other locking fixes for the loop
> device but honestly I don't think that's a stable material - never heard
> of real users hitting problems, only syzkaller could, and we are still
> fixing up some small glitches resulting from that rework...

I tried to backport a number of the loop fixes, and odds are I am the
one that grabbed this.  There are other loop locking fixes in the stable
releases, I don't know if I can just revert this one, let me check...

And yes, I did get some loop bugreports from the Android kernel team as
they are using loop devices in large numbers on the new Android release
for something.  So I think they have already backported a number of
these fixes to their trees, which made me want to push these out to more
people.

Also, given that syzbot has a reproducer for 310ca162d77, are we worst
off if we revert it?

thanks,

greg k-h