linux-kernel - Re: lockup in rb_get_reader

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Message-ID: <1270084982.19685.8151.camel@gandalf.stny.rr.com>
Date:	Wed, 31 Mar 2010 21:23:02 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jiaying Zhang <jiayingz@...gle.com>
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, 2010-03-31 at 18:14 -0700, Jiaying Zhang wrote:
> On Wed, Mar 31, 2010 at 5:28 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > 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?

Nope, unless that write hangs for some reason ;-)

The read may spin waiting for a writer to finish, hence it just can not
preempt a writer in an interrupt context, because then it may spin
forever, because it is blocking the writer. But if a writer is on
another CPU, then all is well. In fact, the ring buffer was designed
with that in mind.

-- Steve

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