hello,
On 07/28/2016 06:22 PM, David Sterba wrote:
On Thu, Jul 28, 2016 at 10:43:15AM +0800, Wang Xiaoguang wrote:
When running fstests generic/068, sometimes we got below deadlock:
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. There still maybe some codes adding
delayed iputs, see below sample race window:
CPU1 | CPU2
|-> freeze_super() |
|-> sync_filesystem(sb); |
| |-> cleaner_kthread()
| | |->
btrfs_delete_unused_bgs()
| | |-> btrfs_remove_chunk()
| | |->
btrfs_remove_block_group()
| | |->
btrfs_add_delayed_iput()
| |
|-> sb->s_writers.frozen = SB_FREEZE_FS; |
|-> sb_wait_write(sb, SB_FREEZE_FS); |
| acquire SB_FREEZE_FS lock. |
| |
|-> btrfs_freeze() |
|-> btrfs_commit_transaction() |
|-> btrfs_run_delayed_iputs() |
| will handle delayed iputs, |
| that means start_transaction() |
| will be called, which will try |
| to get SB_FREEZE_FS lock. |
To fix this issue, introduce a atomic_t fs_frozen to record internally whether
fs has been frozen. If fs has been frozen, we can not handle delayed iputs.
Signed-off-by: Wang Xiaoguang <wangxg.f...@cn.fujitsu.com>
---
v3: we introduce a atomic_t fs_frozen, and if fs_frozen is 1, we can not
handle delayed iputs.
---
fs/btrfs/ctree.h | 2 ++
fs/btrfs/disk-io.c | 1 +
fs/btrfs/super.c | 10 ++++++++++
fs/btrfs/transaction.c | 7 ++++++-
4 files changed, 19 insertions(+), 1 deletion(-)
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 90041a2..9dddaa0 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1091,6 +1091,8 @@ struct btrfs_fs_info {
struct list_head pinned_chunks;
int creating_free_space_tree;
+ /* Use this atomic to record internally whether fs has been frozen */
+ atomic_t fs_frozen;
};
struct btrfs_subvolume_writers {
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 86cad9a..1b85f55 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2621,6 +2621,7 @@ int open_ctree(struct super_block *sb,
atomic_set(&fs_info->qgroup_op_seq, 0);
atomic_set(&fs_info->reada_works_cnt, 0);
atomic64_set(&fs_info->tree_mod_seq, 0);
+ atomic_set(&fs_info->fs_frozen, 0);
fs_info->sb = sb;
fs_info->max_inline = BTRFS_DEFAULT_MAX_INLINE;
fs_info->metadata_ratio = 0;
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 60e7179..c417008 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2216,6 +2216,7 @@ static int btrfs_freeze(struct super_block *sb)
struct btrfs_trans_handle *trans;
struct btrfs_root *root = btrfs_sb(sb)->tree_root;
+ atomic_set(&root->fs_info->fs_frozen, 1);
trans = btrfs_attach_transaction_barrier(root);
if (IS_ERR(trans)) {
/* no transaction, don't bother */
If there's a transaction, we'll call commit a few lines below. With the
frozen status set to 1, the delayed iputs will be skipped
index 948aa18..3187356 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -2277,8 +2277,13 @@ int btrfs_commit_transaction(struct btrfs_trans_handle
*trans,
kmem_cache_free(btrfs_trans_handle_cachep, trans);
+ /*
+ * If fs has been frozen, we can not handle delayed iputs, otherwise
+ * it'll result in deadlock about SB_FREEZE_FS.
+ */
if (current != root->fs_info->transaction_kthread &&
- current != root->fs_info->cleaner_kthread)
+ current != root->fs_info->cleaner_kthread &&
+ !atomic_read(&root->fs_info->fs_frozen))
btrfs_run_delayed_iputs(root);
here. Is that intentional?
Yes. btrfs_run_delayed_iputs() will start new transaction, but if fs is
frozen,
we can not do that. In start_transaction(), it'll call
sb_start_intwrite() trying to
get SB_FREEZE_FS lock, but SB_FREEZE_FS has been locked by fsfreeze.
Also, the use of atomic (probably) works, but just relies on the locked
semantics of the updates although there are no inc/dec operations. An
equivalent would be a bare int with barriers.
Yes, I think a bare int would be enough. Because there are many locks
between "fs_frozen = 1" and
"btrfs_commit_transaction()->btrfs_run_delayed_iputs".
Though I don't have much knowledge about memory barriers :)
Next, I'm not sure if the freeze/thaw could be called multiple times.
Ie. with this patch, if I call freeze twice and thaw once, the
filesystem's frozen status would be 'normal', although the frozen status
as seen by vfs would be 'frozen'. I haven't looked if the vfs magic does
not prevent that completely though.
The first "sudo fsfreeze -f mntpoint" would succeed, but a second
"sudo fsfreeze -f mntpoint" would report:
fsfreeze: mntpoint: freeze failed: Device or resource busy
int freeze_super(struct super_block *sb)
{
int ret;
atomic_inc(&sb->s_active);
down_write(&sb->s_umount);
if (sb->s_writers.frozen != SB_UNFROZEN) {
deactivate_locked_super(sb);
return -EBUSY;
}
...
}
From above codes, we can see that if fs has been frozen,
we can not freeze it anymore.
Regards,
Xiaoguang Wang
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