linux-kernel - Re: PROBLEM: relay - stale data copied to user space

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Message-Id: <1237485040.4752.16.camel@kitka.ibm.com>
Date:	Thu, 19 Mar 2009 18:50:40 +0100
From:	Martin Peschke <mpeschke@...ux.vnet.ibm.com>
To:	Tom Zanussi <tzanussi@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: PROBLEM: relay - stale data copied to user space


On Wed, 2009-03-18 at 23:19 -0500, Tom Zanussi wrote:
> On Wed, 2009-03-18 at 16:07 +0100, Martin Peschke wrote
> > This is my theory:
> > Timing matters. It's a race caused by improper protection of critical
> > sections in a producer-consumer scenario. A bug in the bookkeeping
> > allows a reader to read at a position that is just being written to.
> > 
> 
> It does look consistent with a reader reading an event that's been
> reserved but not yet written, or partially written e.g. if an event
> being written on one cpu was read by another before the first one
> finished.

So this is part of relay's design, and it's up to user space to make
sure that reader and writer are on the same CPU?

> Can you see if the below patch to blktrace userspace helps?

It appears to fix it. I will give it more testing in a larger
environment.

> Or failing that, explicitly using gettid() in place of getpid() in
> sched_setaffinity().  Or, failing that, you had mentioned previously
> that you would try to reproduce the problem on your laptop - were you
> able to do that?  If so, it would help in debugging it further...

This didn't work out. But then, it's a single-CPU machine.

Thanks,
Martin


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