On Wed, 2014-04-16 at 11:26 -0400, Alan Stern wrote:
> On Wed, 16 Apr 2014, Oliver Neukum wrote:
>
> > Hi,
> >
> > I am looking at memory ordering and a question hit me.
> > I was looking at the kfifo code. kfifo_put() has a barrier:
> >
> > )[__kfifo->in & __tmp->kfifo.mask] = \
> > (typeof(*__tmp->type))__val; \
> > smp_wmb(); \
> > __kfifo->in++; \
> >
> > Looking at kfifo_get()
> >
> > __ret = !kfifo_is_empty(__tmp); \
> > if (__ret) { \
> > *(typeof(__tmp->type))__val = \
> > (__is_kfifo_ptr(__tmp) ? \
> >
> > A thought struck me. There is no corresponding barrier. I cannot
> > help myself, but I think there needs to be a smp_read_barrier_depends()
> > between reading kfifo->in (in kfifo_is empty) and reading val.
> > What do you think?
>
> I think you are right.
>
> In addition, the following code in kfifo_get() does this:
>
> *(typeof(__tmp->type))__val = \
> (__is_kfifo_ptr(__tmp) ? \
> ((typeof(__tmp->type))__kfifo->data) : \
> (__tmp->buf) \
> )[__kfifo->out & __tmp->kfifo.mask]; \
> smp_wmb(); \
> __kfifo->out++; \
>
> It looks like the smp_wmb() should really be smp_mb(), because it
> separates the _read_ for val from the _write_ of kfifo->out.
On the third hand, I now think wmb() is sufficient, because
there's also a write to __val. It does depend on the read
of buf[out & mask], but if no CPU does speculative writes
it must be correct.
Regards
Oliver
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