From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Fri, 17 Jan 2003 19:56:45 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Fri, 17 Jan 2003 19:56:44 -0500 Received: from packet.digeo.com ([12.110.80.53]:50307 "EHLO packet.digeo.com") by vger.kernel.org with ESMTP id ; Fri, 17 Jan 2003 19:56:43 -0500 Date: Fri, 17 Jan 2003 17:05:16 -0800 From: Andrew Morton To: Marcelo Tosatti Cc: ext3-users@redhat.com, linux-kernel@vger.kernel.org Subject: [patch 2.4] Fix ext3 scheduling storm and lockup Message-Id: <20030117170516.2e184b82.akpm@digeo.com> X-Mailer: Sylpheed version 0.8.9 (GTK+ 1.2.10; i586-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 18 Jan 2003 01:05:36.0437 (UTC) FILETIME=[B57D9E50:01C2BE8D] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org This patch fixes an inefficiency and potential system lockup in the 2.4 kernel's ext3 filesystem. The problem has been present since 2.4.20-pre5. This patch is applicable to 2.4.20. A copy is at http://www.zip.com.au/~akpm/linux/patches/2.4/2.4.20/ext3-scheduling-storm.patch Anyone who is using tasks which have realtime scheduling policy on ext3 systems should apply this change. Details: At the start of do_get_write_access() we have this logic: repeat: lock_buffer(jh->bh); ... unlock_buffer(jh->bh); ... if (jh->j_list == BJ_Shadow) { sleep_on_buffer(jh->bh); goto repeat; } The problem is that the unlock_buffer() will wake up anyone who is sleeping in the sleep_on_buffer(). So if task A is asleep in sleep_on_buffer() and task B now runs do_get_write_access(), task B will wake task A by accident. Task B will then sleep on the buffer and task A will loop, will run unlock_buffer() and then wake task B. Net effect: the system does 100,000 context switches/sec until I/O completes against the buffer and kjournald changes the value of jh->j_list. Unless task A and task B happen to both have realtime scheduling policy - if they do then kjournald will never run. The state is never cleared and your box locks up. The fix is to not do the `goto repeat;' until the buffer has been taken off the shadow list. So we don't go and wake up the other waiter(s) until they can actually proceed to use the buffer. diff -puN fs/jbd/transaction.c~ext3-scheduling-storm fs/jbd/transaction.c --- 24/fs/jbd/transaction.c~ext3-scheduling-storm 2003-01-16 02:45:19.000000000 -0800 +++ 24-akpm/fs/jbd/transaction.c 2003-01-16 02:45:19.000000000 -0800 @@ -669,7 +669,8 @@ repeat: spin_unlock(&journal_datalist_lock); unlock_journal(journal); /* commit wakes up all shadow buffers after IO */ - sleep_on(&jh2bh(jh)->b_wait); + wait_event(jh2bh(jh)->b_wait, + jh->b_jlist != BJ_Shadow); lock_journal(journal); goto repeat; } _