From a3229f785a16efc330ae96a2de69d27b1a251afe Mon Sep 17 00:00:00 2001 From: Theodore Ts'o Date: Wed, 19 Sep 2012 22:42:36 -0400 Subject: [PATCH] ext4: fix potential deadlock in ext4_nonda_switch() 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. So we can just call writeback_inodes_sb() directly without taking s_umount first. [ 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" Cc: stable@vger.kernel.org --- fs/ext4/inode.c | 4 ++-- fs/fs-writeback.c | 1 + 2 files changed, 3 insertions(+), 2 deletions(-) diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index 0a9a89ec91ff..d4c60f2500bb 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -2449,8 +2449,8 @@ static int ext4_nonda_switch(struct super_block *sb) * 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); + if ((free_blocks < 2 * dirty_blocks) && writeback_in_progress(sb->s_bdi)) + writeback_inodes_sb(sb, WB_REASON_FS_FREE_SPACE); return 0; } diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index be3efc4f64f4..5602d73d4ec7 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) { -- 2.39.5