[PATCH 14/19] quota: protect dqget() from parallels quotaoff via RCU

[Dmitry Monakhov <dmonakhov@openvz.org>]

This allow us to remove dq_state_lock from dqget quota state test.
synchronize_rcu() in dquot_disable() is a bit strage usage for RCU,
but rcu_read_lock will be used in dqget() more properly later to
get make lookup lock-less.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
---
 fs/quota/dquot.c |   32 ++++++++++++++++++++++----------
 1 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
index 771aaab..0d755ef 100644
--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -878,7 +878,7 @@ static struct dquot *get_empty_dquot(struct super_block *sb, int type)
 /*
  * Get reference to dquot
  *
- * Locking is slightly tricky here. We are guarded from parallel quotaoff()
+ * We are guarded from parallel quotaoff() by holding rcu_read_lock
  * destroying our dquot by:
  *   a) checking for quota flags under dq_list_lock and
  *   b) getting a reference to dquot before we release dq_list_lock
@@ -888,17 +888,15 @@ struct dquot *dqget(struct super_block *sb, unsigned int id, int type)
 	struct dquot *dquot = NULL, *empty = NULL;
 	struct quota_info *dqopt = dqopts(sb);
 
-        if (!sb_has_quota_active(sb, type))
-		return NULL;
 we_slept:
-	spin_lock(&dqopt->dq_list_lock);
-	spin_lock(&dqopt->dq_state_lock);
+	rcu_read_lock();
 	if (!sb_has_quota_active(sb, type)) {
-		spin_unlock(&dqopt->dq_state_lock);
-		spin_unlock(&dqopt->dq_list_lock);
-		goto out;
+		rcu_read_unlock();
+		return NULL;
 	}
-	spin_unlock(&dqopt->dq_state_lock);
+	spin_lock(&dqopt->dq_list_lock);
+	/* XXX: Currently RCU used only for synchronization with quotaoff */
+	rcu_read_unlock();
 
 	dquot = find_dquot(sb, id, type);
 	if (!dquot) {
@@ -2113,6 +2111,21 @@ int dquot_disable(struct super_block *sb, int type, unsigned int flags)
 		if (sb_has_quota_loaded(sb, cnt) && !(flags & DQUOT_SUSPENDED))
 			continue;
 
+		toputinode[cnt] = dqopt->files[cnt];
+	}
+	/*
+	 * Wait for all dqget() callers to finish.
+	 */
+	synchronize_rcu();
+
+	/*
+	 * At this moment all quota functions disabled, is is now safe to
+	 * perform final cleanup.
+	 */
+	for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
+
+		if (!toputinode[cnt])
+			continue;
 		/* Note: these are blocking operations */
 		drop_dquot_ref(sb, cnt);
 		invalidate_dquots(sb, cnt);
@@ -2126,7 +2139,6 @@ int dquot_disable(struct super_block *sb, int type, unsigned int flags)
 			dqopt->fmt_ops[cnt]->free_file_info(sb, cnt);
 		put_quota_format(dqopt->info[cnt].dqi_format);
 
-		toputinode[cnt] = dqopt->files[cnt];
 		if (!sb_has_quota_loaded(sb, cnt))
 			dqopt->files[cnt] = NULL;
 		dqopt->info[cnt].dqi_flags = 0;
-- 
1.6.5.2