[PATCH v2 2/7] quota: add new global dquot list releasing_dquots

[Baokun Li <libaokun1@huawei.com>]

Add a new global dquot list that obeys the following rules:

 1). A dquot is added to this list when its last reference count is about
     to be dropped.
 2). The reference count of the dquot in the list is greater than or equal
     to 1 ( due to possible race with dqget()).
 3). When a dquot is removed from this list, a reference count is always
     subtracted, and if the reference count is then 0, the dquot is added
     to the free_dquots list.

This list is used to safely perform the final cleanup before releasing
the last reference count, to avoid various contention issues caused by
performing cleanup directly in dqput(), and to avoid the performance impact
caused by calling synchronize_srcu(&dquot_srcu) directly in dqput(). Here
it is just defining the list and implementing the corresponding operation
function, which we will use later.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
---
 fs/quota/dquot.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
index 108ba9f1e420..a8b43b5b5623 100644
--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -226,12 +226,21 @@ static void put_quota_format(struct quota_format_type *fmt)
 /*
  * Dquot List Management:
  * The quota code uses four lists for dquot management: the inuse_list,
- * free_dquots, dqi_dirty_list, and dquot_hash[] array. A single dquot
- * structure may be on some of those lists, depending on its current state.
+ * releasing_dquots, free_dquots, dqi_dirty_list, and dquot_hash[] array.
+ * A single dquot structure may be on some of those lists, depending on
+ * its current state.
  *
  * All dquots are placed to the end of inuse_list when first created, and this
  * list is used for invalidate operation, which must look at every dquot.
  *
+ * When the last reference of a dquot will be dropped, the dquot will be
+ * added to releasing_dquots. We'd then queue work item which would call
+ * synchronize_srcu() and after that perform the final cleanup of all the
+ * dquots on the list. Both releasing_dquots and free_dquots use the
+ * dq_free list_head in the dquot struct. when a dquot is removed from
+ * releasing_dquots, a reference count is always subtracted, and if
+ * dq_count == 0 at that point, the dquot will be added to the free_dquots.
+ *
  * Unused dquots (dq_count == 0) are added to the free_dquots list when freed,
  * and this list is searched whenever we need an available dquot.  Dquots are
  * removed from the list as soon as they are used again, and
@@ -250,6 +259,7 @@ static void put_quota_format(struct quota_format_type *fmt)
 
 static LIST_HEAD(inuse_list);
 static LIST_HEAD(free_dquots);
+static LIST_HEAD(releasing_dquots);
 static unsigned int dq_hash_bits, dq_hash_mask;
 static struct hlist_head *dquot_hash;
 
@@ -305,6 +315,13 @@ static inline void put_dquot_last(struct dquot *dquot)
 	dqstats_inc(DQST_FREE_DQUOTS);
 }
 
+static inline void put_releasing_dquots(struct dquot *dquot)
+{
+	list_add_tail(&dquot->dq_free, &releasing_dquots);
+	/* dquot will be moved to free_dquots during shrink. */
+	dqstats_inc(DQST_FREE_DQUOTS);
+}
+
 static inline void remove_free_dquot(struct dquot *dquot)
 {
 	if (list_empty(&dquot->dq_free))
-- 
2.31.1