On Tue, Aug 27, 2019 at 10:07:43AM +0800, kaixuxia wrote:
> On 2019/8/27 8:41, Darrick J. Wong wrote:
> > 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...
>
> Right, we will have the ABBA deadlock here if we have to allocate the
> block fist and then put the target_ip on the unlinked list. Of course,
> we need to fix it, but these two deadlock problems have different use
> case and different reasons, maybe it's better that we fix them with
> different patches, and then the subject of this patch need to be
> changed...
> I also can send another patch to fix the new deadlock problem.
Ok. By the way, do you have a quick reproducer that we can put into
xfstests?
--D
>
> >
> > --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
>
> --
> kaixuxia
