hello,
On 07/05/2016 01:35 AM, David Sterba wrote:
On Wed, Jun 29, 2016 at 01:15:10PM +0800, Wang Xiaoguang 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);
So if btrfs is doing freeze job, we should block
btrfs_delete_unused_bgs(), to avoid add delayed iputs.
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.
+ *
Extra line, but I think you intended to write a comment that explains
why the freeze protection is required here :)
Yes, but forgot to... :)
*/
+ __sb_start_write(root->fs_info->sb, SB_FREEZE_WRITE, true);
There's opencoding an existing wrapper sb_start_write, please use it
instead.
OK, I can submit a new version using this wrapper.
Also could you please have a look at my reply to Filipe Manana in
last mail? I suggest another solution, thanks.
Regards,
Xiaoguang Wang
btrfs_delete_unused_bgs(root->fs_info);
+ __sb_end_write(root->fs_info->sb, SB_FREEZE_WRITE);
sleep:
if (!again) {
set_current_state(TASK_INTERRUPTIBLE);
--
2.9.0
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