linux-kernel - [RFC][PATCH v2 10/11] livepatch: Remove klp_synchronize

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Date:   Wed, 29 Sep 2021 17:17:33 +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,
        rostedt@...dmis.org, x86@...nel.org
Subject: [RFC][PATCH v2 10/11] livepatch: Remove klp_synchronize_transition()

The whole premise is broken, any trace usage outside of RCU is a BUG
and should be fixed. Notable all code prior (and a fair bit after)
ct_user_exit() is noinstr and explicitly doesn't allow any tracing.

Use regular RCU, which has the benefit of actually respecting
NOHZ_FULL.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/livepatch/transition.c |   32 +++-----------------------------
 1 file changed, 3 insertions(+), 29 deletions(-)

--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -42,28 +42,6 @@ static void klp_transition_work_fn(struc
 static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
 
 /*
- * This function is just a stub to implement a hard force
- * of synchronize_rcu(). This requires synchronizing
- * tasks even in userspace and idle.
- */
-static void klp_sync(struct work_struct *work)
-{
-}
-
-/*
- * We allow to patch also functions where RCU is not watching,
- * e.g. before ct_user_exit(). We can not rely on the RCU infrastructure
- * to do the synchronization. Instead hard force the sched synchronization.
- *
- * This approach allows to use RCU functions for manipulating func_stack
- * safely.
- */
-static void klp_synchronize_transition(void)
-{
-	schedule_on_each_cpu(klp_sync);
-}
-
-/*
  * The transition to the target patch state is complete.  Clean up the data
  * structures.
  */
@@ -96,7 +74,7 @@ static void klp_complete_transition(void
 		 * func->transition gets cleared, the handler may choose a
 		 * removed function.
 		 */
-		klp_synchronize_transition();
+		synchronize_rcu();
 	}
 
 	klp_for_each_object(klp_transition_patch, obj)
@@ -105,7 +83,7 @@ static void klp_complete_transition(void
 
 	/* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
 	if (klp_target_state == KLP_PATCHED)
-		klp_synchronize_transition();
+		synchronize_rcu();
 
 	read_lock(&tasklist_lock);
 	for_each_process_thread(g, task) {
@@ -168,10 +146,6 @@ noinstr void __klp_update_patch_state(st
  */
 void klp_update_patch_state(struct task_struct *task)
 {
-	/*
-	 * A variant of synchronize_rcu() is used to allow patching functions
-	 * where RCU is not watching, see klp_synchronize_transition().
-	 */
 	preempt_disable_notrace();
 
 	/*
@@ -626,7 +600,7 @@ void klp_reverse_transition(void)
 		clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
 
 	/* Let any remaining calls to klp_update_patch_state() complete */
-	klp_synchronize_transition();
+	synchronize_rcu();
 
 	klp_start_transition();
 }