dquot_scan_active() can race with quota deactivation in
quota_release_workfn() like:

  CPU0 (quota_release_workfn)         CPU1 (dquot_scan_active)
  ==============================      ==============================
  spin_lock(&dq_list_lock);
  list_replace_init(
    &releasing_dquots, &rls_head);
    /* dquot X on rls_head,
       dq_count == 0,
       DQ_ACTIVE_B still set */
  spin_unlock(&dq_list_lock);
  synchronize_srcu(&dquot_srcu);
                                      spin_lock(&dq_list_lock);
                                      list_for_each_entry(dquot,
                                          &inuse_list, dq_inuse) {
                                        /* finds dquot X */
                                        dquot_active(X) -> true
                                        atomic_inc(&X->dq_count);
                                      }
                                      spin_unlock(&dq_list_lock);
  spin_lock(&dq_list_lock);
  dquot = list_first_entry(&rls_head);
  WARN_ON_ONCE(atomic_read(&dquot->dq_count));

The problem is not only a cosmetic one as under memory pressure the
caller of dquot_scan_active() can end up working on freed dquot.

Fix the problem by making sure the dquot is removed from releasing list
when we acquire a reference to it.

Fixes: 869b6ea1609f ("quota: Fix slow quotaoff")
Reported-by: Sam Sun <samsun1006219@gmail.com>
Link: https://lore.kernel.org/all/CAEkJfYPTt3uP1vAYnQ5V2ZWn5O9PLhhGi5HbOcAzyP9vbXyjeg@mail.gmail.com
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/quota/dquot.c         | 38 ++++++++++++++++++++++++++++++--------
 include/linux/quotaops.h |  9 +--------
 2 files changed, 31 insertions(+), 16 deletions(-)

I plan to merge this fix through my tree.

diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
index 376739f6420e..64cf42721496 100644
--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -363,6 +363,31 @@ static inline int dquot_active(struct dquot *dquot)
 	return test_bit(DQ_ACTIVE_B, &dquot->dq_flags);
 }
 
+static struct dquot *__dqgrab(struct dquot *dquot)
+{
+	lockdep_assert_held(&dq_list_lock);
+	if (!atomic_read(&dquot->dq_count))
+		remove_free_dquot(dquot);
+	atomic_inc(&dquot->dq_count);
+	return dquot;
+}
+
+/*
+ * Get reference to dquot when we got pointer to it by some other means. The
+ * dquot has to be active and the caller has to make sure it cannot get
+ * deactivated under our hands.
+ */
+struct dquot *dqgrab(struct dquot *dquot)
+{
+	spin_lock(&dq_list_lock);
+	WARN_ON_ONCE(!dquot_active(dquot));
+	dquot = __dqgrab(dquot);
+	spin_unlock(&dq_list_lock);
+
+	return dquot;
+}
+EXPORT_SYMBOL_GPL(dqgrab);
+
 static inline int dquot_dirty(struct dquot *dquot)
 {
 	return test_bit(DQ_MOD_B, &dquot->dq_flags);
@@ -641,15 +666,14 @@ int dquot_scan_active(struct super_block *sb,
 			continue;
 		if (dquot->dq_sb != sb)
 			continue;
-		/* Now we have active dquot so we can just increase use count */
-		atomic_inc(&dquot->dq_count);
+		__dqgrab(dquot);
 		spin_unlock(&dq_list_lock);
 		dqput(old_dquot);
 		old_dquot = dquot;
 		/*
 		 * ->release_dquot() can be racing with us. Our reference
-		 * protects us from new calls to it so just wait for any
-		 * outstanding call and recheck the DQ_ACTIVE_B after that.
+		 * protects us from dquot_release() proceeding so just wait for
+		 * any outstanding call and recheck the DQ_ACTIVE_B after that.
 		 */
 		wait_on_dquot(dquot);
 		if (dquot_active(dquot)) {
@@ -717,7 +741,7 @@ int dquot_writeback_dquots(struct super_block *sb, int type)
 			/* Now we have active dquot from which someone is
  			 * holding reference so we can safely just increase
 			 * use count */
-			dqgrab(dquot);
+			__dqgrab(dquot);
 			spin_unlock(&dq_list_lock);
 			err = dquot_write_dquot(dquot);
 			if (err && !ret)
@@ -963,9 +987,7 @@ struct dquot *dqget(struct super_block *sb, struct kqid qid)
 		spin_unlock(&dq_list_lock);
 		dqstats_inc(DQST_LOOKUPS);
 	} else {
-		if (!atomic_read(&dquot->dq_count))
-			remove_free_dquot(dquot);
-		atomic_inc(&dquot->dq_count);
+		__dqgrab(dquot);
 		spin_unlock(&dq_list_lock);
 		dqstats_inc(DQST_CACHE_HITS);
 		dqstats_inc(DQST_LOOKUPS);
diff --git a/include/linux/quotaops.h b/include/linux/quotaops.h
index c334f82ed385..f9c0f9d7c9d9 100644
--- a/include/linux/quotaops.h
+++ b/include/linux/quotaops.h
@@ -44,14 +44,7 @@ int dquot_initialize(struct inode *inode);
 bool dquot_initialize_needed(struct inode *inode);
 void dquot_drop(struct inode *inode);
 struct dquot *dqget(struct super_block *sb, struct kqid qid);
-static inline struct dquot *dqgrab(struct dquot *dquot)
-{
-	/* Make sure someone else has active reference to dquot */
-	WARN_ON_ONCE(!atomic_read(&dquot->dq_count));
-	WARN_ON_ONCE(!test_bit(DQ_ACTIVE_B, &dquot->dq_flags));
-	atomic_inc(&dquot->dq_count);
-	return dquot;
-}
+struct dquot *dqgrab(struct dquot *dquot);
 
 static inline bool dquot_is_busy(struct dquot *dquot)
 {
-- 
2.51.0