On Sat, Aug 24, 2019 at 11:45:15AM +0800, kaixuxia wrote:
> When performing rename operation with RENAME_WHITEOUT flag, we will
> hold AGF lock to allocate or free extents in manipulating the dirents
> firstly, and then doing the xfs_iunlink_remove() call last to hold
> AGI lock to modify the tmpfile info, so we the lock order AGI->AGF.
>
> The big problem here is that we have an ordering constraint on AGF
> and AGI locking - inode allocation locks the AGI, then can allocate
> a new extent for new inodes, locking the AGF after the AGI. Hence
> the ordering that is imposed by other parts of the code is AGI before
> AGF. So we get an ABBA deadlock between the AGI and AGF here.
>
> Process A:
> Call trace:
> ? __schedule+0x2bd/0x620
> schedule+0x33/0x90
> schedule_timeout+0x17d/0x290
> __down_common+0xef/0x125
> ? xfs_buf_find+0x215/0x6c0 [xfs]
> down+0x3b/0x50
> xfs_buf_lock+0x34/0xf0 [xfs]
> xfs_buf_find+0x215/0x6c0 [xfs]
> xfs_buf_get_map+0x37/0x230 [xfs]
> xfs_buf_read_map+0x29/0x190 [xfs]
> xfs_trans_read_buf_map+0x13d/0x520 [xfs]
> xfs_read_agf+0xa6/0x180 [xfs]
> ? schedule_timeout+0x17d/0x290
> xfs_alloc_read_agf+0x52/0x1f0 [xfs]
> xfs_alloc_fix_freelist+0x432/0x590 [xfs]
> ? down+0x3b/0x50
> ? xfs_buf_lock+0x34/0xf0 [xfs]
> ? xfs_buf_find+0x215/0x6c0 [xfs]
> xfs_alloc_vextent+0x301/0x6c0 [xfs]
> xfs_ialloc_ag_alloc+0x182/0x700 [xfs]
> ? _xfs_trans_bjoin+0x72/0xf0 [xfs]
> xfs_dialloc+0x116/0x290 [xfs]
> xfs_ialloc+0x6d/0x5e0 [xfs]
> ? xfs_log_reserve+0x165/0x280 [xfs]
> xfs_dir_ialloc+0x8c/0x240 [xfs]
> xfs_create+0x35a/0x610 [xfs]
> xfs_generic_create+0x1f1/0x2f0 [xfs]
> ...
>
> Process B:
> Call trace:
> ? __schedule+0x2bd/0x620
> ? xfs_bmapi_allocate+0x245/0x380 [xfs]
> schedule+0x33/0x90
> schedule_timeout+0x17d/0x290
> ? xfs_buf_find+0x1fd/0x6c0 [xfs]
> __down_common+0xef/0x125
> ? xfs_buf_get_map+0x37/0x230 [xfs]
> ? xfs_buf_find+0x215/0x6c0 [xfs]
> down+0x3b/0x50
> xfs_buf_lock+0x34/0xf0 [xfs]
> xfs_buf_find+0x215/0x6c0 [xfs]
> xfs_buf_get_map+0x37/0x230 [xfs]
> xfs_buf_read_map+0x29/0x190 [xfs]
> xfs_trans_read_buf_map+0x13d/0x520 [xfs]
> xfs_read_agi+0xa8/0x160 [xfs]
> xfs_iunlink_remove+0x6f/0x2a0 [xfs]
> ? current_time+0x46/0x80
> ? xfs_trans_ichgtime+0x39/0xb0 [xfs]
> xfs_rename+0x57a/0xae0 [xfs]
> xfs_vn_rename+0xe4/0x150 [xfs]
> ...
>
> In this patch we move the xfs_iunlink_remove() call to
> before acquiring the AGF lock to preserve correct AGI/AGF locking
> order.
>
> Signed-off-by: kaixuxia <kaixuxia@xxxxxxxxxxx>
> Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>
> ---
> fs/xfs/xfs_inode.c | 83 +++++++++++++++++++++++++++---------------------------
> 1 file changed, 42 insertions(+), 41 deletions(-)
>
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 6467d5e..8ffd44f 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -3282,7 +3282,8 @@ struct xfs_iunlink {
> spaceres);
>
> /*
> - * Set up the target.
> + * Check for expected errors before we dirty the transaction
> + * so we can return an error without a transaction abort.
> */
> if (target_ip == NULL) {
> /*
> @@ -3294,6 +3295,46 @@ struct xfs_iunlink {
> if (error)
> goto out_trans_cancel;
> }
> + } else {
> + /*
> + * If target exists and it's a directory, check that whether
> + * it can be destroyed.
> + */
> + if (S_ISDIR(VFS_I(target_ip)->i_mode) &&
> + (!xfs_dir_isempty(target_ip) ||
> + (VFS_I(target_ip)->i_nlink > 2))) {
> + error = -EEXIST;
> + goto out_trans_cancel;
> + }
> + }
> +
> + /*
> + * Directory entry creation below may acquire the AGF. Remove
> + * the whiteout from the unlinked list first to preserve correct
> + * AGI/AGF locking order. This dirties the transaction so failures
> + * after this point will abort and log recovery will clean up the
> + * mess.
> + *
> + * For whiteouts, we need to bump the link count on the whiteout
> + * inode. After this point, we have a real link, clear the tmpfile
> + * state flag from the inode so it doesn't accidentally get misused
> + * in future.
> + */
> + if (wip) {
> + ASSERT(VFS_I(wip)->i_nlink == 0);
> + error = xfs_iunlink_remove(tp, wip);
> + if (error)
> + goto out_trans_cancel;
> +
> + xfs_bumplink(tp, wip);
> + xfs_trans_log_inode(tp, wip, XFS_ILOG_CORE);
> + VFS_I(wip)->i_state &= ~I_LINKABLE;
> + }
> +
> + /*
> + * Set up the target.
> + */
> + if (target_ip == NULL) {
> /*
> * If target does not exist and the rename crosses
> * directories, adjust the target directory link count
> @@ -3312,22 +3353,6 @@ struct xfs_iunlink {
> }
> } else { /* target_ip != NULL */
> /*
> - * If target exists and it's a directory, check that both
> - * target and source are directories and that target can be
> - * destroyed, or that neither is a directory.
> - */
> - if (S_ISDIR(VFS_I(target_ip)->i_mode)) {
> - /*
> - * Make sure target dir is empty.
> - */
> - if (!(xfs_dir_isempty(target_ip)) ||
> - (VFS_I(target_ip)->i_nlink > 2)) {
> - error = -EEXIST;
> - goto out_trans_cancel;
> - }
> - }
> -
> - /*
> * Link the source inode under the target name.
> * If the source inode is a directory and we are moving
> * it across directories, its ".." entry will be
...will be replaced and then we droplink the target_ip.
Question: Will we have the same ABBA deadlock potential here if we have
to allocate a block from AG 2 to hold the directory entry, but then we
drop target_ip onto the unlinked list, and target_ip was also from AG 2?
We also have to lock the AGI to put things on the unlinked list.
Granted, that's a slightly different use case, but they seem related...
--D
> @@ -3417,30 +3442,6 @@ struct xfs_iunlink {
> if (error)
> goto out_trans_cancel;
>
> - /*
> - * For whiteouts, we need to bump the link count on the whiteout inode.
> - * This means that failures all the way up to this point leave the inode
> - * on the unlinked list and so cleanup is a simple matter of dropping
> - * the remaining reference to it. If we fail here after bumping the link
> - * count, we're shutting down the filesystem so we'll never see the
> - * intermediate state on disk.
> - */
> - if (wip) {
> - ASSERT(VFS_I(wip)->i_nlink == 0);
> - xfs_bumplink(tp, wip);
> - error = xfs_iunlink_remove(tp, wip);
> - if (error)
> - goto out_trans_cancel;
> - xfs_trans_log_inode(tp, wip, XFS_ILOG_CORE);
> -
> - /*
> - * Now we have a real link, clear the "I'm a tmpfile" state
> - * flag from the inode so it doesn't accidentally get misused in
> - * future.
> - */
> - VFS_I(wip)->i_state &= ~I_LINKABLE;
> - }
> -
> xfs_trans_ichgtime(tp, src_dp, XFS_ICHGTIME_MOD | XFS_ICHGTIME_CHG);
> xfs_trans_log_inode(tp, src_dp, XFS_ILOG_CORE);
> if (new_parent)
> --
> 1.8.3.1
>
> --
> kaixuxia
