Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751639Ab0KXOJO (ORCPT ); Wed, 24 Nov 2010 09:09:14 -0500 Received: from 206.83.70.70.ptr.us.xo.net ([206.83.70.70]:60017 "EHLO USMAMAIL.TILERA.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751351Ab0KXOJN (ORCPT ); Wed, 24 Nov 2010 09:09:13 -0500 Message-ID: <4CED1C87.7010500@tilera.com> Date: Wed, 24 Nov 2010 09:09:11 -0500 From: Chris Metcalf User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.12) Gecko/20101027 Thunderbird/3.1.6 MIME-Version: 1.0 To: Cypher Wu CC: , =?ISO-8859-1?Q?Am=E9rico_Wang?= , Eric Dumazet , netdev Subject: Re: [PATCH] arch/tile: fix rwlock so would-be write lockers don't block new readers References: <1289489007.17691.1310.camel@edumazet-laptop> <4CDF1945.8090101@tilera.com> <201011151425.oAFEPU3W005682@farm-0010.internal.tilera.com> <4CEA71AD.5010606@tilera.com> <4CEC2BE7.4060205@tilera.com> In-Reply-To: Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1823 Lines: 35 On 11/23/2010 9:53 PM, Cypher Wu wrote: > 2010/11/24 Chris Metcalf : >> On 11/22/2010 8:36 PM, Cypher Wu wrote: >>> Say, if core A try to write_lock() rwlock and current_ticket_ is 0 and >>> it write next_ticket_ to 1, when it processing the lock, core B try to >>> write_lock() again and write next_ticket_ to 2, then when A >>> write_unlock() it seen that (current_ticket_+1) is not equal to >>> next_ticket_, so it increment current_ticket_, and core B get the >>> lock. If core A try write_lock again before core B write_unlock, it >>> will increment next_ticket_ to 3. And so on. >>> This may rarely happened, I've tested it yesterday for several hours >>> it goes very well under pressure. >> This should be OK when it happens (other than starving out the readers, but >> that was the decision made by doing a ticket lock in the first place). >> Even if we wrap around 255 back to zero on the tickets, the ticket queue >> will work correctly. The key is not to need more than 256 concurrent write >> lock waiters, which we don't. > If we count on that, should we make 'my_ticket_ = (val >> > WR_NEXT_SHIFT) & WR_MASK;' No, it's OK. As the comment for the declaration of "my_ticket_" says, the trailing underscore reminds us that the high bits are garbage, and when we use the value, we do the mask: "((my_ticket_ - curr_) & WR_MASK)". It turned out doing the mask here made the most sense from a code-generation point of view, partly just because of the possibility of the counter wrapping. -- Chris Metcalf, Tilera Corp. http://www.tilera.com -- 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/