Re: lockup in rb_get_reader_page

[Steven Rostedt]

Hi Jiaying,


On Tue, 2010-03-30 at 16:27 -0700, Jiaying Zhang wrote:
> Hi Steven,
> 
> We recently saw some cpu lockups while running kernel tracing.
> The problem started to happen after we synced up our ring buffer
> code with the upstream lockless ring buffer change. It usually
> took many hours and heavy trace load to hit this problem. When
> the lockup happens, the problematic cpu seemed to be in an infinite
> loop of trying to grab the head_page in rb_get_reader_page().
> 
> We would like to check with you on whether this is a known issue.
> If so, do we have a bug fix? If not, do you have any suggestions on
> where we should check?

I'm unaware of any problems with the ring buffer. I've been running the
lockless version for over a year now, and hammering it with very
intensive tracing.

Now, you see this on the reader side. There has been a few recent fixes
that could cause problems when we have multiple readers. Ftrace usage
does not usually encounter multiple readers so I have not had issues.
But Li Zefan had a stress test that did find and trigger the problems.

Are you using the latest ring buffer that is in Linus's tree? Are you
resetting the ring buffer while reading it?

I guess I need to know more exactly what you are doing to understand the
problem.

Thanks,

-- Steve


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