From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754353AbbLIJj4 (ORCPT ); Wed, 9 Dec 2015 04:39:56 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:45175 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754325AbbLIJju (ORCPT ); Wed, 9 Dec 2015 04:39:50 -0500 From: Luis Henriques To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@lists.ubuntu.com Cc: Brian Norris , Luis Henriques Subject: [PATCH 3.16.y-ckt 048/126] mtd: blkdevs: fix potential deadlock + lockdep warnings Date: Wed, 9 Dec 2015 09:36:58 +0000 Message-Id: <1449653896-5236-49-git-send-email-luis.henriques@canonical.com> In-Reply-To: <1449653896-5236-1-git-send-email-luis.henriques@canonical.com> References: <1449653896-5236-1-git-send-email-luis.henriques@canonical.com> X-Extended-Stable: 3.16 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.16.7-ckt21 -stable review patch. If anyone has any objections, please let me know. ------------------ From: Brian Norris commit f3c63795e90f0c6238306883b6c72f14d5355721 upstream. Commit 073db4a51ee4 ("mtd: fix: avoid race condition when accessing mtd->usecount") fixed a race condition but due to poor ordering of the mutex acquisition, introduced a potential deadlock. The deadlock can occur, for example, when rmmod'ing the m25p80 module, which will delete one or more MTDs, along with any corresponding mtdblock devices. This could potentially race with an acquisition of the block device as follows. -> blktrans_open() -> mutex_lock(&dev->lock); -> mutex_lock(&mtd_table_mutex); -> del_mtd_device() -> mutex_lock(&mtd_table_mutex); -> blktrans_notify_remove() -> del_mtd_blktrans_dev() -> mutex_lock(&dev->lock); This is a classic (potential) ABBA deadlock, which can be fixed by making the A->B ordering consistent everywhere. There was no real purpose to the ordering in the original patch, AFAIR, so this shouldn't be a problem. This ordering was actually already present in del_mtd_blktrans_dev(), for one, where the function tried to ensure that its caller already held mtd_table_mutex before it acquired &dev->lock: if (mutex_trylock(&mtd_table_mutex)) { mutex_unlock(&mtd_table_mutex); BUG(); } So, reverse the ordering of acquisition of &dev->lock and &mtd_table_mutex so we always acquire mtd_table_mutex first. Snippets of the lockdep output follow: # modprobe -r m25p80 [ 53.419251] [ 53.420838] ====================================================== [ 53.427300] [ INFO: possible circular locking dependency detected ] [ 53.433865] 4.3.0-rc6 #96 Not tainted [ 53.437686] ------------------------------------------------------- [ 53.444220] modprobe/372 is trying to acquire lock: [ 53.449320] (&new->lock){+.+...}, at: [] del_mtd_blktrans_dev+0x80/0xdc [ 53.457271] [ 53.457271] but task is already holding lock: [ 53.463372] (mtd_table_mutex){+.+.+.}, at: [] del_mtd_device+0x18/0x100 [ 53.471321] [ 53.471321] which lock already depends on the new lock. [ 53.471321] [ 53.479856] [ 53.479856] the existing dependency chain (in reverse order) is: [ 53.487660] -> #1 (mtd_table_mutex){+.+.+.}: [ 53.492331] [] blktrans_open+0x34/0x1a4 [ 53.497879] [] __blkdev_get+0xc4/0x3b0 [ 53.503364] [] blkdev_get+0x108/0x320 [ 53.508743] [] do_dentry_open+0x218/0x314 [ 53.514496] [] path_openat+0x4c0/0xf9c [ 53.519959] [] do_filp_open+0x5c/0xc0 [ 53.525336] [] do_sys_open+0xfc/0x1cc [ 53.530716] [] ret_fast_syscall+0x0/0x1c [ 53.536375] -> #0 (&new->lock){+.+...}: [ 53.540587] [] mutex_lock_nested+0x38/0x3cc [ 53.546504] [] del_mtd_blktrans_dev+0x80/0xdc [ 53.552606] [] blktrans_notify_remove+0x7c/0x84 [ 53.558891] [] del_mtd_device+0x74/0x100 [ 53.564544] [] del_mtd_partitions+0x80/0xc8 [ 53.570451] [] mtd_device_unregister+0x24/0x48 [ 53.576637] [] spi_drv_remove+0x1c/0x34 [ 53.582207] [] __device_release_driver+0x88/0x114 [ 53.588663] [] device_release_driver+0x20/0x2c [ 53.594843] [] bus_remove_device+0xd8/0x108 [ 53.600748] [] device_del+0x10c/0x210 [ 53.606127] [] device_unregister+0xc/0x20 [ 53.611849] [] __unregister+0x10/0x20 [ 53.617211] [] device_for_each_child+0x50/0x7c [ 53.623387] [] spi_unregister_master+0x58/0x8c [ 53.629578] [] release_nodes+0x15c/0x1c8 [ 53.635223] [] __device_release_driver+0x90/0x114 [ 53.641689] [] driver_detach+0xb4/0xb8 [ 53.647147] [] bus_remove_driver+0x4c/0xa0 [ 53.652970] [] SyS_delete_module+0x11c/0x1e4 [ 53.658976] [] ret_fast_syscall+0x0/0x1c [ 53.664621] [ 53.664621] other info that might help us debug this: [ 53.664621] [ 53.672979] Possible unsafe locking scenario: [ 53.672979] [ 53.679169] CPU0 CPU1 [ 53.683900] ---- ---- [ 53.688633] lock(mtd_table_mutex); [ 53.692383] lock(&new->lock); [ 53.698306] lock(mtd_table_mutex); [ 53.704658] lock(&new->lock); [ 53.707946] [ 53.707946] *** DEADLOCK *** Fixes: 073db4a51ee4 ("mtd: fix: avoid race condition when accessing mtd->usecount") Reported-by: Felipe Balbi Tested-by: Felipe Balbi Signed-off-by: Brian Norris Signed-off-by: Luis Henriques --- drivers/mtd/mtd_blkdevs.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c index 7266a318628c..f2af5b91c0e3 100644 --- a/drivers/mtd/mtd_blkdevs.c +++ b/drivers/mtd/mtd_blkdevs.c @@ -199,8 +199,8 @@ static int blktrans_open(struct block_device *bdev, fmode_t mode) if (!dev) return -ERESTARTSYS; /* FIXME: busy loop! -arnd*/ - mutex_lock(&dev->lock); mutex_lock(&mtd_table_mutex); + mutex_lock(&dev->lock); if (dev->open) goto unlock; @@ -224,8 +224,8 @@ static int blktrans_open(struct block_device *bdev, fmode_t mode) unlock: dev->open++; - mutex_unlock(&mtd_table_mutex); mutex_unlock(&dev->lock); + mutex_unlock(&mtd_table_mutex); blktrans_dev_put(dev); return ret; @@ -235,8 +235,8 @@ error_release: error_put: module_put(dev->tr->owner); kref_put(&dev->ref, blktrans_dev_release); - mutex_unlock(&mtd_table_mutex); mutex_unlock(&dev->lock); + mutex_unlock(&mtd_table_mutex); blktrans_dev_put(dev); return ret; } @@ -248,8 +248,8 @@ static void blktrans_release(struct gendisk *disk, fmode_t mode) if (!dev) return; - mutex_lock(&dev->lock); mutex_lock(&mtd_table_mutex); + mutex_lock(&dev->lock); if (--dev->open) goto unlock; @@ -263,8 +263,8 @@ static void blktrans_release(struct gendisk *disk, fmode_t mode) __put_mtd_device(dev->mtd); } unlock: - mutex_unlock(&mtd_table_mutex); mutex_unlock(&dev->lock); + mutex_unlock(&mtd_table_mutex); blktrans_dev_put(dev); }