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Date:	Wed, 06 Aug 2014 13:58:58 -0700
From:	Dave Hansen <dave@...1.net>
To:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	paulmck@...ux.vnet.ibm.com, Steven Rostedt <rostedt@...dmis.org>,
	"Brown, Len" <len.brown@...el.com>
Subject: Re: suspicious RCU usage. (TLB flush tracepoints)

On 08/06/2014 11:18 AM, Dave Jones wrote:
> ===============================
> [ INFO: suspicious RCU usage. ]
> 3.16.0+ #34 Not tainted
> -------------------------------
> include/trace/events/tlb.h:35 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/1/0.
> 
> stack backtrace:
> CPU: 1 PID: 0 Comm: swapper/1 Not tainted 3.16.0+ #34
>  0000000000000001 e7d0f46a57e60fc7 ffff880243357db0 ffffffff8a7f1e37
>  ffff880243360000 ffff880243357de0 ffffffff8a0cc6c5 ffff8801753693f8
>  ffff88023e2e2a40 0000000000000001 ffff88023e2e2a40 ffff880243357e10
> Call Trace:
>  [<ffffffff8a7f1e37>] dump_stack+0x4e/0x7a
>  [<ffffffff8a0cc6c5>] lockdep_rcu_suspicious+0xd5/0x110
>  [<ffffffff8a049f05>] leave_mm+0x1a5/0x200
>  [<ffffffff8a3ec8df>] intel_idle+0x16f/0x190
>  [<ffffffff8a6623da>] cpuidle_enter_state+0x3a/0xd0
>  [<ffffffff8a662557>] cpuidle_enter+0x17/0x20
>  [<ffffffff8a0c719c>] cpu_startup_entry+0x43c/0x800
>  [<ffffffff8a03232d>] start_secondary+0x29d/0x3b0

Wow, this is quite the trainwreck of subsystems.  We've got idle, RCU,
tracing and the VM all fighting with each other.  How fun!

The end result is that we can't use tracepoints in parts of the idle
thread?  That's kinda a bummer.  I'm curious why we don't see this more
widely.  We have a tracepoint *IMMEDIATELY* After one of the
rcu_idle_enter():

> static inline int cpu_idle_poll(void)
> {
>         rcu_idle_enter();
>         trace_cpu_idle_rcuidle(0, smp_processor_id());

Surely there are some more.

The intel_idle and acpi_idle drivers both do this TLB trick, although
the ACPI one is needlessly obfuscated:

	#define acpi_unlazy_tlb(x)      leave_mm(x)

vs the direct call in intel_idle:

	if (state->flags & CPUIDLE_FLAG_TLB_FLUSHED)
		leave_mm(cpu);

Can we just move the leave_mm() to be outside the rcu_idle_enter()?  If
not, I'm just inclined to axe the tracepoint.
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