Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932593Ab2JNOsG (ORCPT ); Sun, 14 Oct 2012 10:48:06 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:46747 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932231Ab2JNOoQ (ORCPT ); Sun, 14 Oct 2012 10:44:16 -0400 Message-Id: <20121014143538.828959405@decadent.org.uk> User-Agent: quilt/0.60-1 Date: Sun, 14 Oct 2012 15:36:10 +0100 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Theodore Tso Subject: [ 037/147] ext4: fix potential deadlock in ext4_nonda_switch() In-Reply-To: <20121014143533.742627615@decadent.org.uk> X-SA-Exim-Connect-IP: 77.75.106.1 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5176 Lines: 133 3.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: Theodore Ts'o commit 00d4e7362ed01987183e9528295de3213031309c upstream. In ext4_nonda_switch(), if the file system is getting full we used to call writeback_inodes_sb_if_idle(). The problem is that we can be holding i_mutex already, and this causes a potential deadlock when writeback_inodes_sb_if_idle() when it tries to take s_umount. (See lockdep output below). As it turns out we don't need need to hold s_umount; the fact that we are in the middle of the write(2) system call will keep the superblock pinned. Unfortunately writeback_inodes_sb() checks to make sure s_umount is taken, and the VFS uses a different mechanism for making sure the file system doesn't get unmounted out from under us. The simplest way of dealing with this is to just simply grab s_umount using a trylock, and skip kicking the writeback flusher thread in the very unlikely case that we can't take a read lock on s_umount without blocking. Also, we now check the cirteria for kicking the writeback thread before we decide to whether to fall back to non-delayed writeback, so if there are any outstanding delayed allocation writes, we try to get them resolved as soon as possible. [ INFO: possible circular locking dependency detected ] 3.6.0-rc1-00042-gce894ca #367 Not tainted ------------------------------------------------------- dd/8298 is trying to acquire lock: (&type->s_umount_key#18){++++..}, at: [] writeback_inodes_sb_if_idle+0x28/0x46 but task is already holding lock: (&sb->s_type->i_mutex_key#8){+.+...}, at: [] generic_file_aio_write+0x5f/0xd3 which lock already depends on the new lock. 2 locks held by dd/8298: #0: (sb_writers#2){.+.+.+}, at: [] generic_file_aio_write+0x56/0xd3 #1: (&sb->s_type->i_mutex_key#8){+.+...}, at: [] generic_file_aio_write+0x5f/0xd3 stack backtrace: Pid: 8298, comm: dd Not tainted 3.6.0-rc1-00042-gce894ca #367 Call Trace: [] ? console_unlock+0x345/0x372 [] print_circular_bug+0x190/0x19d [] __lock_acquire+0x86d/0xb6c [] ? mark_held_locks+0x5c/0x7b [] lock_acquire+0x66/0xb9 [] ? writeback_inodes_sb_if_idle+0x28/0x46 [] down_read+0x28/0x58 [] ? writeback_inodes_sb_if_idle+0x28/0x46 [] writeback_inodes_sb_if_idle+0x28/0x46 [] ext4_nonda_switch+0xe1/0xf4 [] ext4_da_write_begin+0x27/0x193 [] generic_file_buffered_write+0xc8/0x1bb [] __generic_file_aio_write+0x1dd/0x205 [] generic_file_aio_write+0x78/0xd3 [] ext4_file_write+0x480/0x4a6 [] ? __lock_acquire+0x41e/0xb6c [] ? sched_clock_cpu+0x11a/0x13e [] ? trace_hardirqs_off+0xb/0xd [] ? local_clock+0x37/0x4e [] do_sync_write+0x67/0x9d [] ? wait_on_retry_sync_kiocb+0x44/0x44 [] vfs_write+0x7b/0xe6 [] sys_write+0x3b/0x64 [] syscall_call+0x7/0xb Signed-off-by: "Theodore Ts'o" Signed-off-by: Ben Hutchings --- fs/ext4/inode.c | 17 ++++++++++------- fs/fs-writeback.c | 1 + 2 files changed, 11 insertions(+), 7 deletions(-) diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index ca76b5e..0a31197 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -2462,6 +2462,16 @@ static int ext4_nonda_switch(struct super_block *sb) free_blocks = EXT4_C2B(sbi, percpu_counter_read_positive(&sbi->s_freeclusters_counter)); dirty_blocks = percpu_counter_read_positive(&sbi->s_dirtyclusters_counter); + /* + * Start pushing delalloc when 1/2 of free blocks are dirty. + */ + if (dirty_blocks && (free_blocks < 2 * dirty_blocks) && + !writeback_in_progress(sb->s_bdi) && + down_read_trylock(&sb->s_umount)) { + writeback_inodes_sb(sb, WB_REASON_FS_FREE_SPACE); + up_read(&sb->s_umount); + } + if (2 * free_blocks < 3 * dirty_blocks || free_blocks < (dirty_blocks + EXT4_FREECLUSTERS_WATERMARK)) { /* @@ -2470,13 +2480,6 @@ static int ext4_nonda_switch(struct super_block *sb) */ return 1; } - /* - * Even if we don't switch but are nearing capacity, - * start pushing delalloc when 1/2 of free blocks are dirty. - */ - if (free_blocks < 2 * dirty_blocks) - writeback_inodes_sb_if_idle(sb, WB_REASON_FS_FREE_SPACE); - return 0; } diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index be3efc4..5602d73 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -63,6 +63,7 @@ int writeback_in_progress(struct backing_dev_info *bdi) { return test_bit(BDI_writeback_running, &bdi->state); } +EXPORT_SYMBOL(writeback_in_progress); static inline struct backing_dev_info *inode_to_bdi(struct inode *inode) { -- 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/