linux-kernel - Re: lockup in rb_get_reader

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Message-ID: <p2t5df78e1d1003311814q1f270e41o4ebdc1f48659c034@mail.gmail.com>
Date:	Wed, 31 Mar 2010 18:14:57 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	rostedt@...dmis.org
Cc:	Steven Rostedt <srostedt@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Michael Rubin <mrubin@...gle.com>,
	David Sharp <dhsharp@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: lockup in rb_get_reader_page

On Wed, Mar 31, 2010 at 5:28 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 2010-03-30 at 21:01 -0700, Jiaying Zhang wrote:
>
>> I am going to patch rb_get_reader_page() to print out some debugging
>> message when it enters into an infinitely loop. I will keep you updated
>> if I find any interesting info.
>
> I was just thinking about this some more. One restriction to the
> ring_buffer is that a reader can not preempt a writer, which means that
> a read can not happen in an interrupt or NMI. If a read happens there
> and preempts a writer as the writer was moving the head page, it will go
> into an infinite loop waiting for the writer to finish its move.
>
> You can only do reads from interrupt context if the writer disables
> interrupts before reserving and committing the data.
>
> Are you doing reads from interrupt context?
>
No. We are using a user-space reader that reads ring buffer via syscall
or ioctl. But even though we don't read from interrupt context, as I
understand, we can still have a reader on another cpuB reads cpuA's
buffer when the writer on cpuA moves the head page. Will we have
any problem in that case?

Jiaying

> -- Steve
>
>
>
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