On Fri, 18 Apr 2014, Oliver Neukum wrote: > On Thu, 2014-04-17 at 11:50 -0400, Alan Stern wrote: > > On Thu, 17 Apr 2014, Oliver Neukum wrote: > > > > > On Wed, 2014-04-16 at 11:26 -0400, Alan Stern wrote: > > > > > > > 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. > > > > > > But where is kfifo->out read at all? > > > > It is read in kfifo_put(), inside the call to kfifo_is_full() -> > > kfifo_len(). > > I had overlooked that. In that case kfifo_put() also needs > smp_read_barrier_depends()
I don't think so. kfifo_put() uses the read of kfifo->out to determine whether to perform the store; i.e., it won't store the data if the kfifo is already full. No architectures do speculative writes, so the data can never get stored before kfifo->out is read, even without any memory barriers. smp_read_barrier_depends() is meant to prevent speculative _reads_, or any similar mechanism. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
