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Subject: Re: lockup in rb_get_reader_page
From: Steven Rostedt <rostedt@goodmis.org>
Reply-To: rostedt@goodmis.org
To: Jiaying Zhang <jiayingz@google.com>
Cc: Steven Rostedt <srostedt@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       Michael Rubin <mrubin@google.com>, David Sharp <dhsharp@google.com>,
       linux-kernel@vger.kernel.org
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Organization: Kihon Technologies Inc.
Date: Wed, 31 Mar 2010 21:23:02 -0400
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On Wed, 2010-03-31 at 18:14 -0700, Jiaying Zhang wrote:
> On Wed, Mar 31, 2010 at 5:28 PM, Steven Rostedt <rostedt@goodmis.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