On Wed, Jun 29, 2016 at 6:15 AM, Wang Xiaoguang
<wangxg.fnst@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> ---
> 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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