xfs: fix shutdown hang on invalid inode during create

When the new inode verify in xfs_iread() fails, the create
transaction is aborted and a shutdown occurs. The subsequent unmount
then hangs in xfs_wait_buftarg() on a buffer that has an elevated
hold count. Debug showed that it was an AGI buffer getting stuck:

[   22.576147] XFS (vdb): buffer 0x2/0x1, hold 0x2 stuck
[   22.976213] XFS (vdb): buffer 0x2/0x1, hold 0x2 stuck
[   23.376206] XFS (vdb): buffer 0x2/0x1, hold 0x2 stuck
[   23.776325] XFS (vdb): buffer 0x2/0x1, hold 0x2 stuck

The trace of this buffer leading up to the shutdown (trimmed for
brevity) looks like:

xfs_buf_init:        bno 0x2 nblks 0x1 hold 1 caller xfs_buf_get_map
xfs_buf_get:         bno 0x2 len 0x200 hold 1 caller xfs_buf_read_map
xfs_buf_read:        bno 0x2 len 0x200 hold 1 caller xfs_trans_read_buf_map
xfs_buf_iorequest:   bno 0x2 nblks 0x1 hold 1 caller _xfs_buf_read
xfs_buf_hold:        bno 0x2 nblks 0x1 hold 1 caller xfs_buf_iorequest
xfs_buf_rele:        bno 0x2 nblks 0x1 hold 2 caller xfs_buf_iorequest
xfs_buf_iowait:      bno 0x2 nblks 0x1 hold 1 caller _xfs_buf_read
xfs_buf_ioerror:     bno 0x2 len 0x200 hold 1 caller xfs_buf_bio_end_io
xfs_buf_iodone:      bno 0x2 nblks 0x1 hold 1 caller _xfs_buf_ioend
xfs_buf_iowait_done: bno 0x2 nblks 0x1 hold 1 caller _xfs_buf_read
xfs_buf_hold:        bno 0x2 nblks 0x1 hold 1 caller xfs_buf_item_init
xfs_trans_read_buf:  bno 0x2 len 0x200 hold 2 recur 0 refcount 1
xfs_trans_brelse:    bno 0x2 len 0x200 hold 2 recur 0 refcount 1
xfs_buf_item_relse:  bno 0x2 nblks 0x1 hold 2 caller xfs_trans_brelse
xfs_buf_rele:        bno 0x2 nblks 0x1 hold 2 caller xfs_buf_item_relse
xfs_buf_unlock:      bno 0x2 nblks 0x1 hold 1 caller xfs_trans_brelse
xfs_buf_rele:        bno 0x2 nblks 0x1 hold 1 caller xfs_trans_brelse
xfs_buf_trylock:     bno 0x2 nblks 0x1 hold 2 caller _xfs_buf_find
xfs_buf_find:        bno 0x2 len 0x200 hold 2 caller xfs_buf_get_map
xfs_buf_get:         bno 0x2 len 0x200 hold 2 caller xfs_buf_read_map
xfs_buf_read:        bno 0x2 len 0x200 hold 2 caller xfs_trans_read_buf_map
xfs_buf_hold:        bno 0x2 nblks 0x1 hold 2 caller xfs_buf_item_init
xfs_trans_read_buf:  bno 0x2 len 0x200 hold 3 recur 0 refcount 1
xfs_trans_log_buf:   bno 0x2 len 0x200 hold 3 recur 0 refcount 1
xfs_buf_item_unlock: bno 0x2 len 0x200 hold 3 flags DIRTY liflags ABORTED
xfs_buf_unlock:      bno 0x2 nblks 0x1 hold 3 caller xfs_buf_item_unlock
xfs_buf_rele:        bno 0x2 nblks 0x1 hold 3 caller xfs_buf_item_unlock

And that is the AGI buffer from cold cache read into memory to
transaction abort. You can see at transaction abort the bli is dirty
and only has a single reference. The item is not pinned, and it's
not in the AIL. Hence the only reference to it is this transaction.

The problem is that the xfs_buf_item_unlock() call is dropping the
last reference to the xfs_buf_log_item attached to the buffer (which
holds a reference to the buffer), but it is not freeing the
xfs_buf_log_item. Hence nothing will ever release the buffer, and
the unmount hangs waiting for this reference to go away.

The fix is simple - xfs_buf_item_unlock needs to detect the last
reference going away in this case and free the xfs_buf_log_item to
release the reference it holds on the buffer.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_buf.c

@@ -1505,6 +1505,8 @@ xfs_wait_buftarg(
 	while (!list_empty(&btp->bt_lru)) {
 		bp = list_first_entry(&btp->bt_lru, struct xfs_buf, b_lru);
 		if (atomic_read(&bp->b_hold) > 1) {
+			trace_xfs_buf_wait_buftarg(bp, _RET_IP_);
+			list_move_tail(&bp->b_lru, &btp->bt_lru);
 			spin_unlock(&btp->bt_lru_lock);
 			delay(100);
 			goto restart;

fs/xfs/xfs_buf_item.c

@@ -652,7 +652,10 @@ xfs_buf_item_unlock(
 
 	/*
 	 * If the buf item isn't tracking any data, free it, otherwise drop the
-	 * reference we hold to it.
+	 * reference we hold to it. If we are aborting the transaction, this may
+	 * be the only reference to the buf item, so we free it anyway
+	 * regardless of whether it is dirty or not. A dirty abort implies a
+	 * shutdown, anyway.
 	 */
 	clean = 1;
 	for (i = 0; i < bip->bli_format_count; i++) {
@@ -664,7 +667,12 @@ xfs_buf_item_unlock(
 	}
 	if (clean)
 		xfs_buf_item_relse(bp);
-	else
+	else if (aborted) {
+		if (atomic_dec_and_test(&bip->bli_refcount)) {
+			ASSERT(XFS_FORCED_SHUTDOWN(lip->li_mountp));
+			xfs_buf_item_relse(bp);
+		}
+	} else
 		atomic_dec(&bip->bli_refcount);
 
 	if (!hold)

fs/xfs/xfs_trace.h

@@ -341,6 +341,7 @@ DEFINE_BUF_EVENT(xfs_buf_item_relse);
 DEFINE_BUF_EVENT(xfs_buf_item_iodone);
 DEFINE_BUF_EVENT(xfs_buf_item_iodone_async);
 DEFINE_BUF_EVENT(xfs_buf_error_relse);
+DEFINE_BUF_EVENT(xfs_buf_wait_buftarg);
 DEFINE_BUF_EVENT(xfs_trans_read_buf_io);
 DEFINE_BUF_EVENT(xfs_trans_read_buf_shut);