On Mon, Jun 24, 2013 at 01:04:36PM -0400, Steven Rostedt wrote:
> On Mon, 2013-06-24 at 12:51 -0400, Dave Jones wrote:
> > On Mon, Jun 24, 2013 at 12:24:39PM -0400, Steven Rostedt wrote:
> >
> > > > Ah, this is the first victim of my new 'check sanity of nodes during
> > list walks' patch.
> > > > It's doing the same prev->next next->prev checking as list_add and
> > friends.
> > > > I'm looking at getting it into shape for a 3.12 merge after some
> > other preparatory patches
> > > > go into 3.11
> > >
> > > OK, and you may need to make an exception for the ring buffer. To do a
> > > lockless swap out of the reader page for one of the pages in the buffer,
> > > it uses the 2 LSB as flags. Notice the "next=ffff880243288001", that "1"
> > > is a flag that states the next page is the "header" page (next to be
> > > read). We use cmpxchg to update the pages to handle races between the
> > > reader and writer.
> >
> > I just had a plumber come visit to replace my toilet.
> > I think even he would say "dude, gross" about that hack.
>
> Wow, that hack made you so sick you needed to replace your toilet?
>
> Note, the idea of using the 2 LSB bits of pointers came from -rt. Where
> we do the same with the rt_mutex owner.
While I've been spinning wheels trying to reproduce that softlockup bug,
On another machine I've been refining my list-walk debug patch.
I added an ugly "ok, the ringbuffer is playing games with lower two bits"
special case.
But what the hell is going on here ?
next->prev should be prev (ffff88023c6cdd18), but was 00ffff88023c6cdd.
(next=ffff880243288001).
(trace comes from the same ringbuffer code)
Dave
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