linux-kernel - [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU is disabled

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Date:   Mon, 23 Jan 2023 21:50:12 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org
Cc:     will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
        mark.rutland@....com, tglx@...utronix.de, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        seanjc@...gle.com, pbonzini@...hat.com, jgross@...e.com,
        srivatsa@...il.mit.edu, amakhalov@...are.com,
        pv-drivers@...are.com, rostedt@...dmis.org, mhiramat@...nel.org,
        wanpengli@...cent.com, vkuznets@...hat.com,
        boris.ostrovsky@...cle.com, rafael@...nel.org,
        daniel.lezcano@...aro.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        linux-trace-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU is disabled

All RCU disabled code should be noinstr and hence we should never get
here -- when we do, WARN about it and make sure to not actually do
tracing.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/kernel/ftrace.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -646,6 +646,9 @@ void prepare_ftrace_return(unsigned long
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))
 		return;
 
+	if (WARN_ONCE(!rcu_is_watching(), "RCU not on for: %pS\n", (void *)ip))
+		return;
+
 	bit = ftrace_test_recursion_trylock(ip, *parent);
 	if (bit < 0)
 		return;