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Date:	Fri, 21 Sep 2012 17:12:05 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
CC:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	stable@...r.kernel.org
Subject: Re: [PATCH] ext4: fix potential deadlock in ext4_nonda_switch()

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@....edu>
> Cc: stable@...r.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 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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