Re: [BUG] WARNING: at kernel/trace/ring_buffer.c:3420

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Li Zefan wrote:
> While running a ftrace test, a kernel warning showed up. The bug is not so
> easy to reproduce, normally it takes 30 mins to 2 hours.
> 
> After revering 3c05d7482777f15e71bb4cb1ba78dee2800dfec6, I ran the test
> for 15 hours, and the bug seems disapeared.
> 

This bug exhausted us, we found a issue: a tiny window which is
one of causes of this bug. This patch is to close this tiny window.
(bug is not fixed)

From: Lai Jiangshan <laijs@cn.fujitsu.com>

ringbuffer resizing and reseting will increase the ->record_disabled
and then wait until a rcu_shced grace period passes.

Contrarily, testing ->record_disabled should be at the same
preempt disabled critical region as writing into ringbuffer, otherwise
it will leave a window break ringbuffer resizing or reseting.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Reported-by: Li Zefan <lizf@cn.fujitsu.com>
---
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 8c1b2d2..54191d6 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2232,12 +2232,12 @@ ring_buffer_lock_reserve(struct ring_buffer *buffer, unsigned long length)
 	if (ring_buffer_flags != RB_BUFFERS_ON)
 		return NULL;
 
-	if (atomic_read(&buffer->record_disabled))
-		return NULL;
-
 	/* If we are tracing schedule, we don't want to recurse */
 	resched = ftrace_preempt_disable();
 
+	if (atomic_read(&buffer->record_disabled))
+		goto out_nocheck;
+
 	if (trace_recursive_lock())
 		goto out_nocheck;
 
@@ -2469,11 +2469,11 @@ int ring_buffer_write(struct ring_buffer *buffer,
 	if (ring_buffer_flags != RB_BUFFERS_ON)
 		return -EBUSY;
 
-	if (atomic_read(&buffer->record_disabled))
-		return -EBUSY;
-
 	resched = ftrace_preempt_disable();
 
+	if (atomic_read(&buffer->record_disabled))
+		goto out;
+
 	cpu = raw_smp_processor_id();
 
 	if (!cpumask_test_cpu(cpu, buffer->cpumask))