Re: [PATCH 1/2] btrfs: fix fsfreeze hang caused by delayed iputs deal

[Filipe Manana <fdmanana@gmail.com>]

On Wed, Jun 29, 2016 at 6:15 AM, Wang Xiaoguang
<wangxg.fnst@cn.fujitsu.com> wrote:
> When running fstests generic/068, sometimes we got below WARNING:
>   xfs_io          D ffff8800331dbb20     0  6697   6693 0x00000080
>   ffff8800331dbb20 ffff88007acfc140 ffff880034d895c0 ffff8800331dc000
>   ffff880032d243e8 fffffffeffffffff ffff880032d24400 0000000000000001
>   ffff8800331dbb38 ffffffff816a9045 ffff880034d895c0 ffff8800331dbba8
>   Call Trace:
>   [<ffffffff816a9045>] schedule+0x35/0x80
>   [<ffffffff816abab2>] rwsem_down_read_failed+0xf2/0x140
>   [<ffffffff8118f5e1>] ? __filemap_fdatawrite_range+0xd1/0x100
>   [<ffffffff8134f978>] call_rwsem_down_read_failed+0x18/0x30
>   [<ffffffffa06631fc>] ? btrfs_alloc_block_rsv+0x2c/0xb0 [btrfs]
>   [<ffffffff810d32b5>] percpu_down_read+0x35/0x50
>   [<ffffffff81217dfc>] __sb_start_write+0x2c/0x40
>   [<ffffffffa067f5d5>] start_transaction+0x2a5/0x4d0 [btrfs]
>   [<ffffffffa067f857>] btrfs_join_transaction+0x17/0x20 [btrfs]
>   [<ffffffffa068ba34>] btrfs_evict_inode+0x3c4/0x5d0 [btrfs]
>   [<ffffffff81230a1a>] evict+0xba/0x1a0
>   [<ffffffff812316b6>] iput+0x196/0x200
>   [<ffffffffa06851d0>] btrfs_run_delayed_iputs+0x70/0xc0 [btrfs]
>   [<ffffffffa067f1d8>] btrfs_commit_transaction+0x928/0xa80 [btrfs]
>   [<ffffffffa0646df0>] btrfs_freeze+0x30/0x40 [btrfs]
>   [<ffffffff81218040>] freeze_super+0xf0/0x190
>   [<ffffffff81229275>] do_vfs_ioctl+0x4a5/0x5c0
>   [<ffffffff81003176>] ? do_audit_syscall_entry+0x66/0x70
>   [<ffffffff810038cf>] ? syscall_trace_enter_phase1+0x11f/0x140
>   [<ffffffff81229409>] SyS_ioctl+0x79/0x90
>   [<ffffffff81003c12>] do_syscall_64+0x62/0x110
>   [<ffffffff816acbe1>] entry_SYSCALL64_slow_path+0x25/0x25
>
> From this warning, freeze_super() already holds SB_FREEZE_FS, but
> btrfs_freeze() will call btrfs_commit_transaction() again, if
> btrfs_commit_transaction() finds that it has delayed iputs to handle,
> it'll start_transaction(), which will try to get SB_FREEZE_FS lock
> again, then deadlock occurs.
>
> The root cause is that in btrfs, sync_filesystem(sb) does not make
> sure all metadata is updated. See below race window in freeze_super():
>         sync_filesystem(sb);
>                 |
>                 | race window
>                 | In this period, cleaner_kthread() may be scheduled to
>                 | run, and it call btrfs_delete_unused_bgs() which will
>                 | add some delayed iputs.
>                 |
>         sb->s_writers.frozen = SB_FREEZE_FS;
>         sb_wait_write(sb, SB_FREEZE_FS);
>         if (sb->s_op->freeze_fs) {
>                 /* freeze_fs will call btrfs_commit_transaction() */
>                 ret = sb->s_op->freeze_fs(sb);
>

This pseudo diagram also doesn't well the problem.
You should add two timelines for 2 different CPUs/tasks where we see
one locking SB_FREEZE_FS and calling btrfs_freeze()
while the task in the other CPU calls btrfs_commit_transaction(), runs
delayed iputs and then starts/joins a transaction in the
eviction handler.



> So if btrfs is doing freeze job, we should block
> btrfs_delete_unused_bgs(), to avoid add delayed iputs.

Nop, this isn't a real solution.

You can have many other parts adding inodes to the delayed iputs list,
not just btrfs_delete_unused_bgs() (doing it indirectly for the free
space cache inodes).
For example, when an ordered extent completes we add its inode to the
delayed iputs list at btrfs_put_ordered_extent(), called through
btrfs_finish_ordered_io().

thanks



>
> Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
> ---
>  fs/btrfs/disk-io.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 863bf7a..fdbe0df 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -1846,8 +1846,11 @@ static int cleaner_kthread(void *arg)
>                  * after acquiring fs_info->delete_unused_bgs_mutex. So we
>                  * can't hold, nor need to, fs_info->cleaner_mutex when deleting
>                  * unused block groups.
> +                *

Unneeded and unrelated change, please remove it.

>                  */
> +               __sb_start_write(root->fs_info->sb, SB_FREEZE_WRITE, true);
>                 btrfs_delete_unused_bgs(root->fs_info);
> +               __sb_end_write(root->fs_info->sb, SB_FREEZE_WRITE);

A comment explaining why we do this would be valuable, since we never
do these calls except when starting/committing transactions.

>  sleep:
>                 if (!again) {
>                         set_current_state(TASK_INTERRUPTIBLE);
> --
> 2.9.0
>
>
>
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-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."