Re: perf events ring buffer memory barrier on powerpc

[Victor Kaplansky <VICTORK@il.ibm.com>]

Peter Zijlstra <peterz@infradead.org> wrote on 10/30/2013 05:39:31 PM:

> Although I suppose speculative reads are allowed -- they don't have the
> destructive behaviour speculative writes have -- and thus we could in
> fact get reorder issues.

I agree.

>
> But since it is still a dependent load in that we do that @tail vs @head
> comparison before doing other loads, wouldn't a read_barrier_depends()
> be sufficient? Or do we still need a complete rmb?

We need a complete rmb() here IMO. I think there is a fundamental
difference
between load and stores in this aspect. Load are allowed to be hoisted by
compiler or executed speculatively by HW. To prevent load "*(ubuf->data +
tail)"
to be hoisted beyond "ubuf->head" load you would need something like this:

void
ubuf_read(void)
{
        u64 head, tail;

        tail = ubuf->tail;
        head = ACCESS_ONCE(ubuf->head);

        /*
         * Ensure we read the buffer boundaries before the actual buffer
         * data...
         */

        while (tail != head) {
		    smp_read_barrier_depends();         /* for Alpha */
                obj = *(ubuf->data + head - 128);
                /* 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;
}

(note that "head" is part of address calculation of obj load now).

But, even in this demo example some "smp_read_barrier_depends()" before
"obj = *(ubuf->data + head - 100);" is required for architectures
like Alpha. Though, on more sane architectures "smp_read_barrier_depends()"
will be translated to nothing.


Regards,
-- Victor