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

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

Steven Rostedt wrote:
> On Mon, 2010-03-08 at 10:03 -0500, Steven Rostedt wrote:
> 
>>> 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.
>> So the resizing and the resetting need a synchronize_sched() after the
>> disabling of the buffers, right?
> 
> Looking at the code, the synchronize_sched() is already done in
> ring_buffer_resize, and the caller (trace.c:tracing_reset() ) also
> disables the ring buffer and calls synchronize_sched().
> 
> With that, what other window is still opened (after this fix)?
> 

This window is still opened: (RCU vs IDLE vs Tracing)

synchronize_sched() does not protect preempt_disable()/enable() for
idle process. But tracing(function_graph, function) introduce more
preempt_disable()/enable() for idle process. It brings windows.

I bet that this bug is not come from this window.
(I added some strict code to RCU and did stress test,
bug was still occurred.)