On 9/19/12 10:23 PM, Theodore Ts'o wrote:
> 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: [<c02277d4>] writeback_inodes_sb_if_idle+0x28/0x46
>
> but task is already holding lock:
> (&sb->s_type->i_mutex_key#8){+.+...}, at: [<c01ddcce>] 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: [<c01ddcc5>] generic_file_aio_write+0x56/0xd3
> #1: (&sb->s_type->i_mutex_key#8){+.+...}, at: [<c01ddcce>] generic_file_aio_write+0x5f/0xd3
>
> stack backtrace:
> Pid: 8298, comm: dd Not tainted 3.6.0-rc1-00042-gce894ca #367
> Call Trace:
> [<c015b79c>] ? console_unlock+0x345/0x372
> [<c06d62a1>] print_circular_bug+0x190/0x19d
> [<c019906c>] __lock_acquire+0x86d/0xb6c
> [<c01999db>] ? mark_held_locks+0x5c/0x7b
> [<c0199724>] lock_acquire+0x66/0xb9
> [<c02277d4>] ? writeback_inodes_sb_if_idle+0x28/0x46
> [<c06db935>] down_read+0x28/0x58
> [<c02277d4>] ? writeback_inodes_sb_if_idle+0x28/0x46
> [<c02277d4>] writeback_inodes_sb_if_idle+0x28/0x46
> [<c026f3b2>] ext4_nonda_switch+0xe1/0xf4
> [<c0271ece>] ext4_da_write_begin+0x27/0x193
> [<c01dcdb0>] generic_file_buffered_write+0xc8/0x1bb
> [<c01ddc47>] __generic_file_aio_write+0x1dd/0x205
> [<c01ddce7>] generic_file_aio_write+0x78/0xd3
> [<c026d336>] ext4_file_write+0x480/0x4a6
> [<c0198c1d>] ? __lock_acquire+0x41e/0xb6c
> [<c0180944>] ? sched_clock_cpu+0x11a/0x13e
> [<c01967e9>] ? trace_hardirqs_off+0xb/0xd
> [<c018099f>] ? local_clock+0x37/0x4e
> [<c0209f2c>] do_sync_write+0x67/0x9d
> [<c0209ec5>] ? wait_on_retry_sync_kiocb+0x44/0x44
> [<c020a7b9>] vfs_write+0x7b/0xe6
> [<c020a9a6>] sys_write+0x3b/0x64
> [<c06dd4bd>] syscall_call+0x7/0xb
>
> Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> 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 0a9a89e..d4c60f2 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);
Looks to me like this inverts the logic.
We used to write back if idle, now we fire it off if it's already underway.
Shouldn't it be:
+ if ((free_blocks < 2 * dirty_blocks) && !writeback_in_progress(sb->s_bdi))
+ writeback_inodes_sb(sb, WB_REASON_FS_FREE_SPACE);
-Eric
>
> 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)
> {
>
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