Virtualization - May 2023 - [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

On 05/27, Eric W. Biederman wrote:
>
> Looking forward I don't see not asking the worker threads to stop
> for the coredump right now causing any problems in the future.
> So I think we can use this to resolve the coredump issue I spotted.
But we have almost the same problem with exec.

Execing thread will wait for vhost_worker() while vhost_worker will wait for
.release -> vhost_task_stop().

And even O_CLOEXEC won't help, do_close_on_exec() is called after
de_thread().

Or suppose that vhost_worker's sub-thread forks a child with CLONE_FILES...

If we want CLONE_THREAD, I think vhost_worker() should exit after get_signal()
returns SIGKILL. Perhaps it should "disable" vhost_work_queue()
somehow and
flush the pending works on ->work_list before exit, I dunno. But imo it
should
not wait for the final fput().

Oleg.

On 5/29/23 6:19 AM, Oleg Nesterov wrote:
> On 05/27, Eric W. Biederman wrote:
>>
>> Looking forward I don't see not asking the worker threads to stop
>> for the coredump right now causing any problems in the future.
>> So I think we can use this to resolve the coredump issue I spotted.
> 
> But we have almost the same problem with exec.
> 
> Execing thread will wait for vhost_worker() while vhost_worker will wait
for
> .release -> vhost_task_stop().
For this type of case, what is the goal or correct behavior in the end?

When get_signal returns true we can code things like you mention below and
clean up the task_struct. However, we now have a non-functioning vhost device
open and just sitting around taking up memory and it can't do any IO.

For this type of case, do we expect just not to crash/hang, or was this new
exec'd thread suppose to be able to use the vhost device?

I would normally say it probably wants to use the vhost device still. However,
I don't think this comes up so just not hanging might be ok. Before 6.4-rc1,
we ignored signals so it would have worked if we are concerned about a possible
regression if this was a common thing.


> 
> And even O_CLOEXEC won't help, do_close_on_exec() is called after
de_thread().
> 
> Or suppose that vhost_worker's sub-thread forks a child with
CLONE_FILES...
You mean the vhost_task's task/thread doing a function that does a
copy_process
right? That type of thing is not needed. I can add a check in vhost_task_create
for this so new code doesn't try to do it. I don't think it will come up
that some
code vhost is using will call kernel_thread/copy_process directly since those
calls are so rare and the functions are not exported to modules.

> 
> If we want CLONE_THREAD, I think vhost_worker() should exit after
get_signal()
> returns SIGKILL. Perhaps it should "disable" vhost_work_queue()
somehow and
> flush the pending works on ->work_list before exit, I dunno. But imo it
should
> not wait for the final fput().
> 
> Oleg.
>

On 5/29/23 6:19 AM, Oleg Nesterov wrote:
> If we want CLONE_THREAD, I think vhost_worker() should exit after
get_signal()
> returns SIGKILL. Perhaps it should "disable" vhost_work_queue()
somehow and
> flush the pending works on ->work_list before exit, I dunno. But imo it
should
> not wait for the final fput().
Just a FYI, I coded this. It's pre-RFC quality. It's invasive. If we
want to go
this route then we have to do a temp hack for 6.4 or revert then do this route
for 6.5 or 6.6 (vhost devs will need time to review and we need time to full
test).