Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966085Ab2EORbp (ORCPT ); Tue, 15 May 2012 13:31:45 -0400 Received: from merlin.infradead.org ([205.233.59.134]:36422 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758281Ab2EORbn convert rfc822-to-8bit (ORCPT ); Tue, 15 May 2012 13:31:43 -0400 Message-ID: <1337103096.27694.94.camel@twins> Subject: Re: [RFC][PATCH RT] rwsem_rt: Another (more sane) approach to mulit reader rt locks From: Peter Zijlstra To: Steven Rostedt Cc: LKML , RT , Thomas Gleixner , Clark Williams Date: Tue, 15 May 2012 19:31:36 +0200 In-Reply-To: <1337102732.14207.336.camel@gandalf.stny.rr.com> References: <1337090625.14207.304.camel@gandalf.stny.rr.com> <1337094383.27694.62.camel@twins> <1337096542.14207.315.camel@gandalf.stny.rr.com> <1337102732.14207.336.camel@gandalf.stny.rr.com> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.2- Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1460 Lines: 34 On Tue, 2012-05-15 at 13:25 -0400, Steven Rostedt wrote: > On Tue, 2012-05-15 at 11:42 -0400, Steven Rostedt wrote: > > On Tue, 2012-05-15 at 17:06 +0200, Peter Zijlstra wrote: > > > On Tue, 2012-05-15 at 10:03 -0400, Steven Rostedt wrote: > > > > > > > > where readers may nest (the same task may grab the same rwsem for > > > > read multiple times), but only one task may hold the rwsem at any > > > > given > > > > time (for read or write). > > > > > > Humm, that sounds iffy, rwsem isn't a recursive read lock only rwlock_t > > > is. > > > > In that case, current -rt is broken. As it has it being a recursive lock > > (without my patch). Nah not broken, just pointless. A recursive lock that's not used recursively is fine. > > Why wouldn't it be recursive. If two different tasks are allowed to grab > a read lock at the same time, why can't the same task grab a read lock > twice? As long as it releases it the same amount of times. > > Now you can't grab a read lock if you have the write lock. rwsem is fifo-fair, if a writer comes in between the second read acquisition (even by the same task) would block and you'd be a deadlock since the write won't succeed since you're still holding a reader. -- 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/