Subject: Re: perf events ring buffer memory barrier on powerpc

From: "Paul E. McKenney" <[email-masked]>
Date: Sat, 2 Nov 2013 10:28:40 -0700

On Fri, Nov 01, 2013 at 04:25:42PM +0200, Victor Kaplansky wrote:
> "Paul E. McKenney" <[email-masked]> wrote on 10/31/2013
> 08:40:15 AM:
> 
> > > void ubuf_read(void)
> > > {
> > >    u64 head, tail;
> > >
> > >    tail = ACCESS_ONCE(ubuf->tail);
> > >    head = ACCESS_ONCE(ubuf->head);
> > >
> > >    /*
> > >     * Ensure we read the buffer boundaries before the actual buffer
> > >     * data...
> > >     */
> > >    smp_rmb(); /* C, matches with B */
> > >
> > >    while (tail != head) {
> > >       obj = ubuf->data + tail;
> > >       /* process obj */
> > >       tail += obj->size;
> > >       tail %= ubuf->size;
> > >    }
> > >
> > >    /*
> > >     * Ensure all data reads are complete before we issue the
> > >     * ubuf->tail update; once that update hits, kbuf_write() can
> > >     * observe and overwrite data.
> > >     */
> > >    smp_mb(); /* D, matches with A */
> > >
> > >    ubuf->tail = tail;
> > > }
> 
> > > Could we replace A and C with an smp_read_barrier_depends()?
> >
> > C, yes, given that you have ACCESS_ONCE() on the fetch from ->tail
> > and that the value fetch from ->tail feeds into the address used for
> > the "obj =" assignment.
> 
> No! You must to have a full smp_rmb() at C. The race on the reader side
> is not between fetch of @tail and read from address pointed by @tail.
> The real race here is between a fetch of @head and read of obj from
> memory pointed by @tail.

I believe you are in fact correct, good catch.

							Thanx, Paul