From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S263574AbTDNRQA (for ); Mon, 14 Apr 2003 13:16:00 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S263576AbTDNRQA (for ); Mon, 14 Apr 2003 13:16:00 -0400 Received: from users.ccur.com ([208.248.32.211]:52579 "HELO rudolph.ccur.com") by vger.kernel.org with SMTP id S263574AbTDNRP4 (for ); Mon, 14 Apr 2003 13:15:56 -0400 Date: Mon, 14 Apr 2003 13:27:31 -0400 From: Joe Korty To: Robert Love Cc: linux-kernel@vger.kernel.org Subject: [PATCH] 2.4 preemption bug in bh_kmap_irq Message-ID: <20030414172730.GA17451@rudolph.ccur.com> Reply-To: Joe Korty Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.4i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org [I submitted this bug to Alan some time ago, he agreed it was a problem but felt that it should be fixed in the 2.4 preemption patch.] Hi Robert, Everyone, The bh_kmap_irq/bh_kunmap_irq functions are broken in 2.4.21-pre5. However no symptoms occur unless the preemption patch is applied. The bug is that bh_map_irq *conditionally* calls kmap_atomic (which disables preemption as one of its functions), while bh_unmap_irq *unconditionally* calls kunmap_atomic (which enables it). This imbalance results in a occasional off-by-one preempt_count, which in turn causes IDE PIO mode interrupt code (specifically, read_intr) to erronously invoke preempt_schedule while at interrupt level. The below patch compiles and boots ide=nodma on my preempt 2.4 kernel on the one motherboard that had the problem. Before this patch, the kernel would not even boot for that motherboard. I also applied and test booted a pure 2.4.21-pre5 kernel with this patch. The patch implements my preference for simplicity, so you may want to take some other approach if maximal performance is what you want. Joe --- include/linux/highmem.h.orig 2003-03-12 05:01:56.000000000 -0500 +++ include/linux/highmem.h 2003-03-12 16:07:04.000000000 -0500 @@ -33,22 +33,10 @@ { unsigned long addr; - __save_flags(*flags); - - /* - * could be low - */ - if (!PageHighMem(bh->b_page)) - return bh->b_data; - - /* - * it's a highmem page - */ - __cli(); + local_irq_save(*flags); addr = (unsigned long) kmap_atomic(bh->b_page, KM_BH_IRQ); - if (addr & ~PAGE_MASK) - BUG(); + BUG_ON (addr & ~PAGE_MASK); return (char *) addr + bh_offset(bh); } @@ -58,7 +46,7 @@ unsigned long ptr = (unsigned long) buffer & PAGE_MASK; kunmap_atomic((void *) ptr, KM_BH_IRQ); - __restore_flags(*flags); + local_irq_restore(*flags); } #else /* CONFIG_HIGHMEM */ >>From alan@lxorguk.ukuu.org.uk Wed Mar 12 22:19:43 2003 Return-Path: Received: from exchange.ccur.com by rudolph.ccur.com (8.6.10/CCC-4.1) id WAA17543; Wed, 12 Mar 2003 22:19:43 GMT Received: by exchange.ccur.com with Internet Mail Service (5.5.2653.19) id <1J8VJBZT>; Wed, 12 Mar 2003 17:19:43 -0500 Received: from irongate.swansea.linux.org.uk (pc2-cwma1-4-cust86.swan.cable.ntl.com [213.105.254.86]) by exchange.ccur.com with SMTP (Microsoft Exchange Internet Mail Service Version 5.5.2653.13) id 1J8VJBZS; Wed, 12 Mar 2003 17:19:38 -0500 Received: from irongate.swansea.linux.org.uk (localhost [127.0.0.1]) by irongate.swansea.linux.org.uk (8.12.7/8.12.7) with ESMTP id h2CNRVYf024118; Wed, 12 Mar 2003 23:27:32 GMT Received: (from alan@localhost) by irongate.swansea.linux.org.uk (8.12.7/8.12.7/Submit) id h2CNRTqm024116; Wed, 12 Mar 2003 23:27:29 GMT X-Authentication-Warning: irongate.swansea.linux.org.uk: alan set sender to alan@lxorguk.ukuu.org.uk using -f Subject: Re: [PATCH] bug in 2.4 bh_kmap_irq() breaks IDE under preempt patch From: Alan Cox To: Joe Korty Cc: Linux Kernel Mailing List In-Reply-To: <200303122213.WAA17415@rudolph.ccur.com> References: <200303122213.WAA17415@rudolph.ccur.com> Content-Type: text/plain Content-Transfer-Encoding: 7bit Message-Id: <1047511647.23902.29.camel@irongate.swansea.linux.org.uk> Mime-Version: 1.0 X-Mailer: Ximian Evolution 1.2.1 (1.2.1-4) Date: 12 Mar 2003 23:27:28 +0000 Status: RO Content-Length: 406 Lines: 9 On Wed, 2003-03-12 at 22:13, Joe Korty wrote: > The bug is that bh_map_irq *conditionally* calls kmap_atomic (which > disables preemption as one of its functions), while bh_unmap_irq > *unconditionally* calls kunmap_atomic (which enables it). This Thats a pre-empt bug ont a bh_map_irq bug. I'm glad you've found it however. It explains a few things and will be useful for people wanting pre-empt 2.4 .