[PATCH 4/4] tracing: Remove RCU work arounds from stack tracer

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>

While debugging some RCU issues with the stack tracer, it was discovered
that the problem was much more than with the stack tracer itself, but with
the saving of the stack trace, which could happen from any WARN() as well.
The problem was fixed within kernel_text_address().

One of the bugs that was discovered was that the stack tracer called
rcu_enter_irq() unconditionally. Paul McKenney said that could cause issues
as well. Instead of adding logic to only call rcu_enter_irq() if RCU is not
watching from within the stack tracer, since the core issue has been fixed
(within save_stack_trace()), we can simply remove all the logic in the stack
tracer that deals with RCU work arounds.

Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 0be964be0 ("module: Sanitize RCU usage and locking")
Suggested-by: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 kernel/trace/trace_stack.c | 15 ---------------
 1 file changed, 15 deletions(-)

diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index a4df67cbc711..49cb41412eec 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -96,23 +96,9 @@ check_stack(unsigned long ip, unsigned long *stack)
 	if (in_nmi())
 		return;
 
-	/*
-	 * There's a slight chance that we are tracing inside the
-	 * RCU infrastructure, and rcu_irq_enter() will not work
-	 * as expected.
-	 */
-	if (unlikely(rcu_irq_enter_disabled()))
-		return;
-
 	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.
-	 */
-	rcu_irq_enter();
-
 	/* In case another CPU set the tracer_frame on us */
 	if (unlikely(!frame_size))
 		this_size -= tracer_frame;
@@ -205,7 +191,6 @@ check_stack(unsigned long ip, unsigned long *stack)
 	}
 
  out:
-	rcu_irq_exit();
 	arch_spin_unlock(&stack_trace_max_lock);
 	local_irq_restore(flags);
 }
-- 
2.13.2