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@xxxxxxxxxx> > --- > 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
