Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()
From: Peter Zijlstra <peterz@infradead.org>
Date: 2020-02-18 20:07:38
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On Thu, Feb 13, 2020 at 03:44:44PM -0500, Joel Fernandes wrote:
quoted
quoted
That _should_ already be the case today. That is, if we end up in a tracer and in_nmi() is unreliable we're already screwed anyway.
I removed the static from rcu_nmi_enter()/exit() as it is called from outside, that makes it build now. Updated below is Paul's diff. I also added NOKPROBE_SYMBOL() to rcu_nmi_exit() to match rcu_nmi_enter() since it seemed asymmetric.
quoted hunk ↗ jump to hunk
+__always_inline void rcu_nmi_exit(void) { struct rcu_data *rdp = this_cpu_ptr(&rcu_data);@@ -651,25 +653,15 @@ static __always_inline void rcu_nmi_exit_common(bool irq) trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks)); WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */ - if (irq) + if (!in_nmi()) rcu_prepare_for_idle(); rcu_dynticks_eqs_enter(); - if (irq) + if (!in_nmi()) rcu_dynticks_task_enter(); }
Boris and me have been going over the #MC code (and finding loads of
'interesting' code) and ran into ist_enter(), whish has the following
code:
/*
* We might have interrupted pretty much anything. In
* fact, if we're a machine check, we can even interrupt
* NMI processing. We don't want in_nmi() to return true,
* but we need to notify RCU.
*/
rcu_nmi_enter();
Which, to me, sounds all sorts of broken. The IST (be it #DB or #MC) can
happen while we're holding all sorts of locks. This must be an NMI-like
context.