Re: [PATCH 2/2] vhost: check for pending livepatches from vhost worker kthreads

[Petr Mladek <pmladek@suse.com>]

On Fri 2023-01-20 16:12:22, Seth Forshee (DigitalOcean) wrote:
> Livepatch relies on stack checking of sleeping tasks to switch kthreads,
> so a busy kthread can block a livepatch transition indefinitely. We've
> seen this happen fairly often with busy vhost kthreads.

To be precise, it would be "indefinitely" only when the kthread never
sleeps.

But yes. I believe that the problem is real. It might be almost
impossible to livepatch some busy kthreads, especially when they
have a dedicated CPU.


> Add a check to call klp_switch_current() from vhost_worker() when a
> livepatch is pending. In testing this allowed vhost kthreads to switch
> immediately when they had previously blocked livepatch transitions for
> long periods of time.
> 
> Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> ---
>  drivers/vhost/vhost.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index cbe72bfd2f1f..d8624f1f2d64 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -366,6 +367,9 @@ static int vhost_worker(void *data)
>  			if (need_resched())
>  				schedule();
>  		}
> +
> +		if (unlikely(klp_patch_pending(current)))
> +			klp_switch_current();

I suggest to use the following intead:

		if (unlikely(klp_patch_pending(current)))
			klp_update_patch_state(current);

We already use this in do_idle(). The reason is basically the same.
It is almost impossible to livepatch the idle task when a CPU is
very idle.

klp_update_patch_state(current) does not check the stack.
It switches the task immediately.

It should be safe because the kthread never leaves vhost_worker().
It means that the same kthread could never re-enter this function
and use the new code.

Best Regards,
Petr