Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759192AbXELQEa (ORCPT ); Sat, 12 May 2007 12:04:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755807AbXELQEY (ORCPT ); Sat, 12 May 2007 12:04:24 -0400 Received: from mail.screens.ru ([213.234.233.54]:38957 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751750AbXELQEX (ORCPT ); Sat, 12 May 2007 12:04:23 -0400 Date: Sat, 12 May 2007 20:04:28 +0400 From: Oleg Nesterov To: Peter Zijlstra Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Ingo Molnar , Thomas Gleixner , Nick Piggin Subject: Re: [PATCH 1/2] scalable rw_mutex Message-ID: <20070512160428.GA173@tv-sign.ru> References: <20070511131541.992688403@chello.nl> <20070511132321.895740140@chello.nl> <20070511230023.GA449@tv-sign.ru> <1178977276.6810.59.camel@twins> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1178977276.6810.59.camel@twins> User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1918 Lines: 68 On 05/12, Peter Zijlstra wrote: > > +static inline void rw_mutex_readers_dec(struct rw_mutex *rw_mutex) > +{ > + percpu_counter_dec(&rw_mutex->readers); > + smp_wmb(); > +} > > +void rw_mutex_read_unlock(struct rw_mutex *rw_mutex) > +{ > + rw_mutex_readers_dec(rw_mutex); > + /* > + * on the slow path; > + * nudge the writer waiting for the last reader to go away > + */ > + if (unlikely(rw_mutex_reader_slow(rw_mutex))) > + rw_mutex_writer_wake(rw_mutex); > +} > > +void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass) > +{ > + mutex_lock_nested(&rw_mutex->write_mutex, subclass); > + > + /* > + * block new readers > + */ > + mutex_lock_nested(&rw_mutex->read_mutex, subclass); > + rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_SLOW); > + /* > + * and wait for all current readers to go away > + */ > + rw_mutex_writer_wait(rw_mutex, (rw_mutex_readers(rw_mutex) == 0)); > +} I think this is still not right, but when it comes to barriers we need a true expert (Paul cc-ed). this code roughly does (the only reader does unlock) READER WRITER readers = 0; state = 1; wmb(); wmb(); CHECK(state != 0) CHECK(readers == 0) We need to ensure that we can't miss both CHECKs. Either reader should see RW_MUTEX_READER_SLOW, o writer sees "readers == 0" and does not sleep. In that case both barriers should be converted to smp_mb(). There was a _long_ discussion about STORE-MB-LOAD behaviour, and experts seem to believe everething is ok. Another question. Isn't it possible to kill rw_mutex->status ? I have a vague feeling you can change the code so that rw_mutex_reader_slow() <=> "->waiter != NULL" , but I am not sure. Oleg. - 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/