linux-kernel - Re: lockup in rb_get_reader

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Message-Id: <1269995724.22564.58.camel@localhost.localdomain>
Date:	Tue, 30 Mar 2010 20:35:24 -0400
From:	Steven Rostedt <srostedt@...hat.com>
To:	Jiaying Zhang <jiayingz@...gle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Michael Rubin <mrubin@...gle.com>,
	David Sharp <dhsharp@...gle.com>, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: lockup in rb_get_reader_page

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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