[2/2] xfs: release the dquot buf outside of qli_lock

Commit Message

Darrick J. Wong Dec. 19, 2024, 7:20 p.m. UTC

From: Darrick J. Wong <[e-mail]>

Lai Yi reported a lockdep complaint about circular locking:

 Chain exists of:
   &lp->qli_lock --> &bch->bc_lock --> &l->lock

  Possible unsafe locking scenario:

        CPU0                    CPU1
        ----                    ----
   lock(&l->lock);
                                lock(&bch->bc_lock);
                                lock(&l->lock);
   lock(&lp->qli_lock);

I /think/ the problem here is that xfs_dquot_attach_buf during
quotacheck will release the buffer while it's holding the qli_lock.
Because this is a cached buffer, xfs_buf_rele_cached takes b_lock before
decrementing b_hold.  Other threads have taught lockdep that a locking
dependency chain is bp->b_lock -> bch->bc_lock -> l(ru)->lock; and that
another chain is l(ru)->lock -> lp->qli_lock.  Hence we do not want to
take b_lock while holding qli_lock.

Reported-by: [e-mail]
Cc: <[e-mail]> # v6.13-rc3
Fixes: ca378189fdfa89 ("xfs: convert quotacheck to attach dquot buffers")
Tested-by: [e-mail]
Signed-off-by: "Darrick J. Wong" <[e-mail]>
---
 fs/xfs/xfs_dquot.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

Comments

Christoph Hellwig Dec. 20, 2024, 6:29 a.m. UTC | #1

Yes, doing a xfs_buf_rele under any kind of low-level lock is probably
a bad idea:

Reviewed-by: Christoph Hellwig <[e-mail]>

Patch

diff --git a/fs/xfs/xfs_dquot.c b/fs/xfs/xfs_dquot.c
index f11d475898f280..576b7755b1f1fc 100644
--- a/fs/xfs/xfs_dquot.c
+++ b/fs/xfs/xfs_dquot.c
@@ -1315,7 +1315,8 @@  xfs_dquot_read_buf(
 
 /*
  * Attach a dquot buffer to this dquot to avoid allocating a buffer during a
- * dqflush, since dqflush can be called from reclaim context.
+ * dqflush, since dqflush can be called from reclaim context.  Caller must hold
+ * the dqlock.
  */
 int
 xfs_dquot_attach_buf(
@@ -1336,13 +1337,16 @@  xfs_dquot_attach_buf(
 			return error;
 
 		/*
-		 * Attach the dquot to the buffer so that the AIL does not have
-		 * to read the dquot buffer to push this item.
+		 * Hold the dquot buffer so that we retain our ref to it after
+		 * detaching it from the transaction, then give that ref to the
+		 * dquot log item so that the AIL does not have to read the
+		 * dquot buffer to push this item.
 		 */
 		xfs_buf_hold(bp);
+		xfs_trans_brelse(tp, bp);
+
 		spin_lock(&qlip->qli_lock);
 		lip->li_buf = bp;
-		xfs_trans_brelse(tp, bp);
 	}
 	qlip->qli_dirty = true;
 	spin_unlock(&qlip->qli_lock);