linux-kernel - [RFC][PATCH 7/7] livepatch,context_tracking: Avoid disturbing NOHZ

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Date:   Wed, 22 Sep 2021 13:05:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     gor@...ux.ibm.com, jpoimboe@...hat.com, jikos@...nel.org,
        mbenes@...e.cz, pmladek@...e.com, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        joe.lawrence@...hat.com, fweisbec@...il.com, tglx@...utronix.de,
        hca@...ux.ibm.com, svens@...ux.ibm.com, sumanthk@...ux.ibm.com,
        live-patching@...r.kernel.org, paulmck@...nel.org
Subject: [RFC][PATCH 7/7] livepatch,context_tracking: Avoid disturbing NOHZ_FULL tasks

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

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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;