Messages in this thread | | | Date | Wed, 16 Apr 2014 11:26:53 -0400 (EDT) | From | Alan Stern <> | Subject | Re: question on read_barrier_depends |
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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.
Alan Stern
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