Re: [PATCH 3/3] ring-buffer: add design document

Frederic Weisbecker
2009-06-15T00:06:17Z

On Sun, Jun 14, 2009 at 08:39:03AM -0400, Steven Rostedt wrote:
>
> On Sun, 14 Jun 2009, Frederic Weisbecker wrote:
> > > > > +
> > > > > +Now the writer can update the head page. This is also why the head page must
> > > > > +remain in UPDATE and only reset by the outer most writer. This prevents
> > > > > +the reader from seeing the incorrect head page.
> > > > > +
> > > > > +
> > > > > +(first writer)
> > > > > +
> > > > > + A B tail page
> > > > > + | | |
> > > > > + v v v
> > > > > + +---+ +---+ +---+ +---+
> > > > > +<---| |--->| |--->| |--->| |-H->
> > > > > +--->| |<---| |<---| |<---| |<---
> > > > > + +---+ +---+ +---+ +---+
> > > >
> > > >
> > > > Even more tricky!
> > > >
> > > > I just have a stupid question: why can't this be done
> > > > only through HEAD and NORMAL flags?
> > > >
> > > > There is something certainly very obvious that I'm missing
> > > > with the point of the UPDATE flag.
> > >
> > > If you can demonstrate how to do the above lockless with just HEAD and
> > > NORMAL, then sure, I'm all ears ;-)
> > >
> > > When we switch the HEAD to UPDATE, we stop the reader from moving forward
> > > and being another thing to handle while we move the HEAD forward. A reader
> > > does a cmpxchg to move the head too, and that cmpxchg will always fail if
> > > the pointer is has UPDATE set. The reader will just spin until it
> > > succeeds.
> >
> >
> > Aah, so it's here to protect against paralell readers from another cpu
> > reading the current cpu buffer, right?
>
> Not readers, but reader. The reader side uses locks to serialize the
> accesses. The writer side is lockless. But it is here to protect the
> writers against a reader on another CPU.
>
> -- Steve

Ok, my problem is that I've read this doc having in mind that the reader
we are talking about is always reading the buffer on the current cpu,
then a writer always preempt it. I forgot that a reader may also read
from another cpu.

Now I understand.

Thanks.