Messages in this thread | | | Subject | Re: question on read_barrier_depends | From | Oliver Neukum <> | Date | Mon, 21 Apr 2014 11:40:59 +0200 |
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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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