linux-kernel - Fix suspicious RCU usage in msr tracing.

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Date:	Mon, 14 Mar 2016 21:20:54 -0400
From:	Dave Jones <davej@...emonkey.org.uk>
To:	x86@...nel.org
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Fix suspicious RCU usage in msr tracing.

Since 4.4, I've been able to trigger this occasionally:

===============================
[ INFO: suspicious RCU usage. ]
4.5.0-rc7-think+ #3 Not tainted
-------------------------------
./arch/x86/include/asm/msr-trace.h:47 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

RCU used illegally from idle CPU!
rcu_scheduler_active = 1, debug_locks = 1
RCU used illegally from extended quiescent state!
no locks held by swapper/3/0.

stack backtrace:
CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.5.0-rc7-think+ #3
 ffffffff92f821e0 1f3e5c340597d7fc ffff880468e07f10 ffffffff92560c2a
 ffff880462145280 0000000000000001 ffff880468e07f40 ffffffff921376a6
 ffffffff93665ea0 0000cc7c876d28da 0000000000000005 ffffffff9383dd60
Call Trace:
 <IRQ>  [<ffffffff92560c2a>] dump_stack+0x67/0x9d
 [<ffffffff921376a6>] lockdep_rcu_suspicious+0xe6/0x100
 [<ffffffff925ae7a7>] do_trace_write_msr+0x127/0x1a0
 [<ffffffff92061c83>] native_apic_msr_eoi_write+0x23/0x30
 [<ffffffff92054408>] smp_trace_call_function_interrupt+0x38/0x360
 [<ffffffff92d1ca60>] trace_call_function_interrupt+0x90/0xa0
 <EOI>  [<ffffffff92ac5124>] ? cpuidle_enter_state+0x1b4/0x520
 [<ffffffff92ac54e7>] cpuidle_enter+0x17/0x20
 [<ffffffff9212a984>] cpu_startup_entry+0x4e4/0x610
 [<ffffffff9212a4a0>] ? default_idle_call+0x60/0x60
 [<ffffffff921908e4>] ? clockevents_config_and_register+0x64/0x70
 [<ffffffff92055843>] start_secondary+0x283/0x390
 [<ffffffff920555c0>] ? set_cpu_sibling_map+0x970/0x970

Back in January, Andi suggested swapping the order in which
we ack the APIC irq, so it too is protected by rcu.
There was no follow-up, and it seems to work for me.

Cc: Andi Kleen <ak@...ux.intel.com>
Signed-off-by: Dave Jones <davej@...emonkey.org.uk>

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index c80f6b6f3da2..e8c4fba52d3d 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -644,8 +644,8 @@ static inline void entering_irq(void)
 
 static inline void entering_ack_irq(void)
 {
-	ack_APIC_irq();
 	entering_irq();
+	ack_APIC_irq();
 }
 
 static inline void ipi_entering_ack_irq(void)