From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id 5BF40B6F14 for ; Tue, 24 Aug 2010 08:02:16 +1000 (EST) Subject: Re: 64-bit ppc rwsem From: Benjamin Herrenschmidt To: Arnd Bergmann In-Reply-To: <201008231544.16422.arnd@arndb.de> References: <1282107803.22370.173.camel@pasglop> <1282195403.22370.296.camel@pasglop> <20100818.222925.233689776.davem@davemloft.net> <201008231544.16422.arnd@arndb.de> Content-Type: text/plain; charset="UTF-8" Date: Tue, 24 Aug 2010 08:01:25 +1000 Message-ID: <1282600885.22370.453.camel@pasglop> Mime-Version: 1.0 Cc: linuxppc-dev@lists.ozlabs.org, paulus@au.ibm.com, linux-kernel@vger.kernel.org, sparclinux@vger.kernel.org, akpm@linux-foundation.org, torvalds@linux-foundation.org, David Miller List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Mon, 2010-08-23 at 15:44 +0200, Arnd Bergmann wrote: > > * Alpha has an optimization for the uniprocessor case, where the atomic > instructions get turned into nonatomic additions. The spinlock based > version uses no locks on UP but disables interrupts for reasons I don't > understand (nothing running at interrupt time should try to access an rwsem). > Should the generic version do the same as Alpha? I've seen drivers in the past do trylocks at interrupt time ... tho I agree it sucks. > * Is there any architecture that would still benefit from having a separate > rwsem implementation? AFAICT all the remaining ones are just variations of > the same concept of using cmpxchg (or xadd in case of x86), which is what > atomics typically end up doing anyway. It depends how sensitive rwsems are. The "generic" variant based on atomic's and cmpxchg on powerpc is sub-optimal in the sense that it has stronger memory barriers that would be necessary (atomic_inc_return for example has both acquire and release). But that vs. one more pile of inline asm, we decided it wasn't hot enough a spot for us to care back then. Cheers, Ben.