[RFC][PATCH 7/7] livepatch,context_tracking: Avoid disturbing NOHZ_FULL tasks

Peter Zijlstra
2021-09-22T11:11:32Z

When a task is stuck in NOHZ_FULL usermode, we can simply mark the
livepatch state complete.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/livepatch/transition.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -270,13 +270,24 @@ static int klp_check_task(struct task_st
{
int ret;

- if (task_curr(task))
+ if (task_curr(task)) {
+ if (context_tracking_state_cpu(task_cpu(task)) == CONTEXT_USER) {
+ /*
+ * If we observe the CPU being in USER context, they
+ * must issue an smp_mb() before doing much kernel
+ * space and as such will observe the patched state,
+ * mark it clean.
+ */
+ goto complete;
+ }
return -EBUSY;
+ }

ret = klp_check_stack(task, arg);
if (ret)
return ret;

+complete:
clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
task->patch_state = klp_target_state;
return 0;