[PATCH] tracing: Have stack tracer call rcu_nmi_enter() only when RCU is not watching

[Steven Rostedt]

Testing some other code, this RCU splat triggered:

 =============================
 WARNING: suspicious RCU usage
 4.13.0-rc7-test+ #117 Tainted: G        W
 -----------------------------
 arch/x86/kernel/traps.c:305 entry code didn't wake RCU!

 other info that might help us debug this:

 RCU used illegally from idle CPU!
 rcu_scheduler_active = 2, 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 Tainted: G        W       4.13.0-rc7-test+ #117
 Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v03.03 07/14/2016
 Call Trace:
  dump_stack+0x86/0xcf
  lockdep_rcu_suspicious+0xc5/0x100
  do_error_trap+0x125/0x130
  ? do_error_trap+0x5/0x130
  ? trace_hardirqs_off_thunk+0x1a/0x1c
  ? do_invalid_op+0x5/0x30
  do_invalid_op+0x20/0x30
  invalid_op+0x1e/0x30
 RIP: 0010:module_assert_mutex_or_preempt+0x34/0x40
 RSP: 0018:ffffc900006abc58 EFLAGS: 00010046
 RAX: 0000000000000000 RBX: ffffffffa000a077 RCX: 0000000000000002
 RDX: 0000000000000000 RSI: 00000000ffffffff RDI: 0000000000000046
 RBP: ffffc900006abc58 R08: ffffc900006abf40 R09: 0000000000000000
 R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
 R13: 0000000000000000 R14: ffff8801188d8040 R15: ffffffff81ed5720
  ? 0xffffffffa000a077
  ? module_assert_mutex_or_preempt+0x30/0x40
  __module_address+0x2c/0xf0
  ? 0xffffffffa000a077
  __module_text_address+0x12/0x60
  ? 0xffffffffa000a077
  is_module_text_address+0x1f/0x50
  ? 0xffffffffa000a077
  __kernel_text_address+0x30/0x90
  unwind_get_return_address+0x1f/0x30
  __save_stack_trace+0x83/0xd0
  ? 0xffffffffa000a077
  ? rcu_dynticks_eqs_exit+0x5/0x40
  save_stack_trace+0x1b/0x20
  check_stack+0xf8/0x2f0
  ? rcu_dynticks_eqs_enter+0x30/0x30
  stack_trace_call+0x6e/0x80
  0xffffffffa000a077
  ? ftrace_graph_caller+0x78/0xa8
  ? rcu_dynticks_eqs_exit+0x5/0x40
  rcu_dynticks_eqs_exit+0x5/0x40
  rcu_idle_exit+0xdf/0xf0
  ? rcu_dynticks_eqs_exit+0x5/0x40
  ? rcu_idle_exit+0xdf/0xf0
  do_idle+0x128/0x200
  cpu_startup_entry+0x1d/0x20
  start_secondary+0x108/0x130
  secondary_startup_64+0x9f/0x9f

What happened, was that the CPU was coming out of idle, and
rcu_dynticks_eqs_exit() was traced by the stack tracer. The stack was bigger
than the current recorded max stack so the stack tracer triggered a stack
trace calling save_stack_trace(). As save_stack_trace() performs RCU logic,
the stack tracer called rcu_irq_enter() before calling save_stack_trace().
There is two problems here.

1) rcu_irq_enter() is called unconditionally

Talking with Paul McKenney, it should only be called if rcu_is_watching()
returns false.

2) rcu_irq_enter() appears to not function well in this location of the RCU
   internal logic (tracing rcu_dynticks_eqs_exit()), and causes this bug.

The solution is two fold.

1) check if RCU is watching, and only wake RCU if it is not
   (solves the first issue)

2) call rcu_nmi_enter() instead of rcu_irq_enter(). It makes more sense as
   tracing acts more like NMIs and not IRQs (can be called when interrupts
   are disabled. (solves the second issue).

Suggested-by: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 kernel/trace/trace_stack.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index a4df67c..2462e11 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -77,6 +77,7 @@ check_stack(unsigned long ip, unsigned long *stack)
 {
 	unsigned long this_size, flags; unsigned long *p, *top, *start;
 	static int tracer_frame;
+	bool no_rcu;
 	int frame_size = ACCESS_ONCE(tracer_frame);
 	int i, x;
 
@@ -104,14 +105,18 @@ check_stack(unsigned long ip, unsigned long *stack)
 	if (unlikely(rcu_irq_enter_disabled()))
 		return;
 
+	no_rcu = !rcu_is_watching();
+
 	local_irq_save(flags);
 	arch_spin_lock(&stack_trace_max_lock);
 
 	/*
 	 * RCU may not be watching, make it see us.
 	 * The stack trace code uses rcu_sched.
+	 * Note, tracing acts more like an NMI than an IRQ.
 	 */
-	rcu_irq_enter();
+	if (no_rcu)
+		rcu_nmi_enter();
 
 	/* In case another CPU set the tracer_frame on us */
 	if (unlikely(!frame_size))
@@ -205,7 +210,8 @@ check_stack(unsigned long ip, unsigned long *stack)
 	}
 
  out:
-	rcu_irq_exit();
+	if (no_rcu)
+		rcu_nmi_exit();
 	arch_spin_unlock(&stack_trace_max_lock);
 	local_irq_restore(flags);
 }
-- 
2.9.5