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Date:   Tue, 18 Feb 2020 18:31:50 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Tony Luck <tony.luck@intel.com>, x86-ml <x86@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
Subject: [RFC] #MC mess
Message-ID: <20200218173150.GK14449@zn.tnic>
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Ok,

so Peter raised this question on IRC today, that the #MC handler needs
to disable all kinds of tracing/kprobing and etc exceptions happening
while handling an #MC. And I guess we can talk about supporting some
exceptions but #MC is usually nasty enough to not care about tracing
when former happens.

So how about this trivial first stab of using the big hammer and simply
turning off stuff? The nmi_enter()/nmi_exit() thing still needs debating
because ist_enter() already does rcu_nmi_enter() and I'm not sure
whether any of the context tracking would still be ok with that.

Anything else I'm missing? It is likely...

Thx.

---
diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index 2c4f949611e4..6dff97c53310 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -1214,7 +1214,7 @@ static void __mc_scan_banks(struct mce *m, struct mce *final,
  * MCE broadcast. However some CPUs might be broken beyond repair,
  * so be always careful when synchronizing with others.
  */
-void do_machine_check(struct pt_regs *regs, long error_code)
+void notrace do_machine_check(struct pt_regs *regs, long error_code)
 {
 	DECLARE_BITMAP(valid_banks, MAX_NR_BANKS);
 	DECLARE_BITMAP(toclear, MAX_NR_BANKS);
@@ -1251,6 +1251,10 @@ void do_machine_check(struct pt_regs *regs, long error_code)
 	if (__mc_check_crashing_cpu(cpu))
 		return;
 
+	hw_breakpoint_disable();
+	static_key_disable(&__tracepoint_read_msr.key);
+	tracing_off();
+
 	ist_enter(regs);
 
 	this_cpu_inc(mce_exception_count);
@@ -1360,6 +1364,7 @@ void do_machine_check(struct pt_regs *regs, long error_code)
 	ist_exit(regs);
 }
 EXPORT_SYMBOL_GPL(do_machine_check);
+NOKPROBE_SYMBOL(do_machine_check);
 
 #ifndef CONFIG_MEMORY_FAILURE
 int memory_failure(unsigned long pfn, int flags)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette