Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161426AbWHJQXK (ORCPT ); Thu, 10 Aug 2006 12:23:10 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1161425AbWHJQXK (ORCPT ); Thu, 10 Aug 2006 12:23:10 -0400 Received: from sd291.sivit.org ([194.146.225.122]:11269 "EHLO sd291.sivit.org") by vger.kernel.org with ESMTP id S1161424AbWHJQXI (ORCPT ); Thu, 10 Aug 2006 12:23:08 -0400 Subject: Re: [PATCH] memory ordering in __kfifo primitives From: Stelian Pop To: paulmck@us.ibm.com Cc: Mike Christie , linux-kernel@vger.kernel.org, akpm@osdl.org, paulus@au1.ibm.com, anton@au1.ibm.com, open-iscsi@googlegroups.com, pradeep@us.ibm.com, mashirle@us.ibm.com In-Reply-To: <20060810161129.GF1298@us.ibm.com> References: <20060810001823.GA3026@us.ibm.com> <20060810003310.GA3071@us.ibm.com> <44DAC892.7000100@cs.wisc.edu> <20060810134135.GB1298@us.ibm.com> <1155220013.1108.4.camel@localhost.localdomain> <20060810153915.GE1298@us.ibm.com> <1155224842.5393.13.camel@localhost.localdomain> <20060810161129.GF1298@us.ibm.com> Content-Type: text/plain; charset=ISO-8859-15 Date: Thu, 10 Aug 2006 18:23:04 +0200 Message-Id: <1155226984.5393.26.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.6.1 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2265 Lines: 67 Le jeudi 10 ao?t 2006 ? 09:11 -0700, Paul E. McKenney a ?crit : > On Thu, Aug 10, 2006 at 05:47:22PM +0200, Stelian Pop wrote: > > Le jeudi 10 ao?t 2006 ? 08:39 -0700, Paul E. McKenney a ?crit : > > > On Thu, Aug 10, 2006 at 04:26:53PM +0200, Stelian Pop wrote: > > > > Le jeudi 10 ao?t 2006 ? 06:41 -0700, Paul E. McKenney a ?crit : > > > > > > > > > I am happy to go either way -- the patch with the memory barriers > > > > > (which does have the side-effect of slowing down kfifo_get() and > > > > > kfifo_put(), by the way), or a patch removing the comments saying > > > > > that it is OK to invoke __kfifo_get() and __kfifo_put() without > > > > > locking. > > > > > > > > > > Any other thoughts on which is better? (1) the memory barriers or > > > > > (2) requiring the caller hold appropriate locks across calls to > > > > > __kfifo_get() and __kfifo_put()? > > > > > > > > If someone wants to use explicit locking, he/she can go with kfifo_get() > > > > instead of the __ version. > > > > > > However, the kfifo_get()/kfifo_put() interfaces use the internal lock, > > > > ... and the internal lock can be supplied by the user at kfifo_alloc() > > time. > > Would that really work for them? Looks to me like it would result > in self-deadlock if they passed in session->lock. Yeah, it will deadlock if the lock is already taken before calling __kfifo_get and __kfifo_put. > Or did you have something else in mind for them? What I had in mind is to replace all occurences of: kfifo_alloc(..., NULL); ... spin_lock(&session->lock) __kfifo_get() spin_unlock() with the simpler: kfifo_alloc(..., &session->lock) ... kfifo_get() As for the occurences of: ... spin_lock(&session->lock) do_something(); __kifo_get(); well, there is not much we can do about them... Let's take this problem differently: is a memory barrier cheaper than a spinlock ? If the answer is yes as I suspect, why should the kfifo API force the user to take a spinlock ? Stelian. -- Stelian Pop - 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/