"Paul E. McKenney" [off-list ref] wrote on 10/31/2013
08:16:02 AM:

quoted

BTW, it is why you also don't need ACCESS_ONCE() around @tail, but only
around
@head read.

Just to be sure, that we are talking about the same code - I was
considering
ACCESS_ONCE() around @tail in point AAA in the following example from
Documentation/circular-buffers.txt for CONSUMER:

        unsigned long head = ACCESS_ONCE(buffer->head);
        unsigned long tail = buffer->tail;      /* AAA */

        if (CIRC_CNT(head, tail, buffer->size) >= 1) {
                /* read index before reading contents at that index */
                smp_read_barrier_depends();

                /* extract one item from the buffer */
                struct item *item = buffer[tail];

                consume_item(item);

                smp_mb(); /* finish reading descriptor before incrementing
tail */

                buffer->tail = (tail + 1) & (buffer->size - 1); /* BBB */
        }

If you omit the ACCESS_ONCE() calls around @tail, the compiler is within
its rights to combine adjacent operations and also to invent loads and
stores, for example, in cases of register pressure.

Right. And I was completely aware about these possible transformations when
said that ACCESS_ONCE() around @tail in point AAA is redundant. Moved, or
even
completely dismissed reads of @tail in consumer code, are not a problem at
all,
since @tail is written exclusively by CONSUMER side.

It is also within
its rights to do piece-at-a-time loads and stores, which might sound
unlikely, but which can actually has happened when the compiler figures
out exactly what is to be stored at compile time, especially on hardware
that only allows small immediate values.

As for writes to @tail, the ACCESS_ONCE around @tail at point AAA,
doesn't prevent in any way an imaginary super-optimizing compiler
from moving around the store to @tail (which appears in the code at point
BBB).

It is why ACCESS_ONCE at point AAA is completely redundant.

-- Victor