From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760364Ab0GWPGO (ORCPT ); Fri, 23 Jul 2010 11:06:14 -0400 Received: from smtp.nokia.com ([192.100.105.134]:52445 "EHLO mgw-mx09.nokia.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760261Ab0GWPGD (ORCPT ); Fri, 23 Jul 2010 11:06:03 -0400 From: Artem Bityutskiy To: Jens Axboe Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCHv4 02/14] writeback: fix possible race when creating bdi threads Date: Fri, 23 Jul 2010 18:05:42 +0300 Message-Id: <1279897554-1526-3-git-send-email-dedekind1@gmail.com> X-Mailer: git-send-email 1.7.1.1 In-Reply-To: <1279897554-1526-1-git-send-email-dedekind1@gmail.com> References: <1279897554-1526-1-git-send-email-dedekind1@gmail.com> X-OriginalArrivalTime: 23 Jul 2010 15:05:48.0816 (UTC) FILETIME=[88B47100:01CB2A78] X-Nokia-AV: Clean Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Artem Bityutskiy This patch fixes a very unlikely race condition on the bdi forker thread error path: when bdi thread creation fails, 'bdi->wb.task' may contain the error code for a short period of time. If at the same time someone submits a work to this bdi, we can end up with an oops 'bdi_queue_work()' while executing 'wake_up_process(wb->task)'. This patch fixes the issue by introducing a temporary variable 'task' and storing the possible error code there, so that 'wb->task' would never take erroneous values. Note, this race is very unlikely and I never hit it, so it is theoretical, but nevertheless worth fixing. This patch also merges 2 comments which were previously separate. Signed-off-by: Artem Bityutskiy Reviewed-by: Christoph Hellwig --- mm/backing-dev.c | 28 +++++++++++----------------- 1 files changed, 11 insertions(+), 17 deletions(-) diff --git a/mm/backing-dev.c b/mm/backing-dev.c index 4e9ed2a..327e36d 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -331,8 +331,8 @@ static int bdi_forker_thread(void *ptr) set_user_nice(current, 0); for (;;) { + struct task_struct *task; struct backing_dev_info *bdi, *tmp; - struct bdi_writeback *wb; /* * Temporary measure, we want to make sure we don't see @@ -383,29 +383,23 @@ static int bdi_forker_thread(void *ptr) list_del_init(&bdi->bdi_list); spin_unlock_bh(&bdi_lock); - wb = &bdi->wb; - wb->task = kthread_run(bdi_writeback_thread, wb, "flush-%s", - dev_name(bdi->dev)); - /* - * If thread creation fails, then readd the bdi to - * the pending list and force writeout of the bdi - * from this forker thread. That will free some memory - * and we can try again. - */ - if (IS_ERR(wb->task)) { - wb->task = NULL; - + task = kthread_run(bdi_writeback_thread, &bdi->wb, "flush-%s", + dev_name(bdi->dev)); + if (IS_ERR(task)) { /* - * Add this 'bdi' to the back, so we get - * a chance to flush other bdi's to free - * memory. + * If thread creation fails, then readd the bdi back to + * the list and force writeout of the bdi from this + * forker thread. That will free some memory and we can + * try again. Add it to the tail so we get a chance to + * flush other bdi's to free memory. */ spin_lock_bh(&bdi_lock); list_add_tail(&bdi->bdi_list, &bdi_pending_list); spin_unlock_bh(&bdi_lock); bdi_flush_io(bdi); - } + } else + bdi->wb.task = task; } return 0; -- 1.7.1.1