Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161828AbaDPP0z (ORCPT ); Wed, 16 Apr 2014 11:26:55 -0400 Received: from iolanthe.rowland.org ([192.131.102.54]:42583 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1161602AbaDPP0y (ORCPT ); Wed, 16 Apr 2014 11:26:54 -0400 Date: Wed, 16 Apr 2014 11:26:53 -0400 (EDT) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: Oliver Neukum cc: linux-kernel@vger.kernel.org Subject: Re: question on read_barrier_depends In-Reply-To: <1397634429.21891.13.camel@linux-fkkt.site> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/