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?
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.
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.
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