Re: question on read_barrier_depends

[Oliver Neukum <oliver@neukum.org>]

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