[PATCH v2 4/4] fs/dcache: Protect negative dentry pruning from racing with umount

[Waiman Long]

The negative dentry pruning is done on a specific super_block set
in the ndblk.prune_sb variable. If the super_block is also being
un-mounted concurrently, the content of the super_block may no longer
be valid.

To protect against such racing condition, a new lock is added to
the ndblk structure to synchronize the negative dentry pruning and
umount operation. This is a regular spinlock as the pruning operation
can be quite time consuming.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 fs/dcache.c | 42 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 39 insertions(+), 3 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index c2ea876..a3159f3 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -139,11 +139,13 @@ struct dentry_stat_t dentry_stat = {
 static long neg_dentry_nfree_init __read_mostly; /* Free pool initial value */
 static struct {
 	raw_spinlock_t nfree_lock;
+	spinlock_t prune_lock;		/* Lock for protecting pruning */
 	long nfree;			/* Negative dentry free pool */
 	struct super_block *prune_sb;	/* Super_block for pruning */
 	int neg_count, prune_count;	/* Pruning counts */
 } ndblk ____cacheline_aligned_in_smp;
 
+static void clear_prune_sb_for_umount(struct super_block *sb);
 static void prune_negative_dentry(struct work_struct *work);
 static DECLARE_DELAYED_WORK(prune_neg_dentry_work, prune_negative_dentry);
 
@@ -1323,6 +1325,7 @@ void shrink_dcache_sb(struct super_block *sb)
 {
 	long freed;
 
+	clear_prune_sb_for_umount(sb);
 	do {
 		LIST_HEAD(dispose);
 
@@ -1353,7 +1356,8 @@ static enum lru_status dentry_negative_lru_isolate(struct list_head *item,
 	 * list.
 	 */
 	if ((ndblk.neg_count >= NEG_DENTRY_BATCH) ||
-	    (ndblk.prune_count >= NEG_DENTRY_BATCH)) {
+	    (ndblk.prune_count >= NEG_DENTRY_BATCH) ||
+	    !READ_ONCE(ndblk.prune_sb)) {
 		ndblk.prune_count = 0;
 		return LRU_STOP;
 	}
@@ -1408,15 +1412,24 @@ static enum lru_status dentry_negative_lru_isolate(struct list_head *item,
 static void prune_negative_dentry(struct work_struct *work)
 {
 	int freed;
-	struct super_block *sb = READ_ONCE(ndblk.prune_sb);
+	struct super_block *sb;
 	LIST_HEAD(dispose);
 
-	if (!sb)
+	/*
+	 * The prune_lock is used to protect negative dentry pruning from
+	 * racing with concurrent umount operation.
+	 */
+	spin_lock(&ndblk.prune_lock);
+	sb = READ_ONCE(ndblk.prune_sb);
+	if (!sb) {
+		spin_unlock(&ndblk.prune_lock);
 		return;
+	}
 
 	ndblk.neg_count = ndblk.prune_count = 0;
 	freed = list_lru_walk(&sb->s_dentry_lru, dentry_negative_lru_isolate,
 			      &dispose, NEG_DENTRY_BATCH);
+	spin_unlock(&ndblk.prune_lock);
 
 	if (freed)
 		shrink_dentry_list(&dispose);
@@ -1433,6 +1446,27 @@ static void prune_negative_dentry(struct work_struct *work)
 		WRITE_ONCE(ndblk.prune_sb, NULL);
 }
 
+/*
+ * This is called before an umount to clear ndblk.prune_sb if it
+ * matches the given super_block.
+ */
+static void clear_prune_sb_for_umount(struct super_block *sb)
+{
+	if (likely(READ_ONCE(ndblk.prune_sb) != sb))
+		return;
+	WRITE_ONCE(ndblk.prune_sb, NULL);
+	/*
+	 * Need to wait until an ongoing pruning operation, if present,
+	 * is completed.
+	 *
+	 * Clearing ndblk.prune_sb will hasten the completion of pruning.
+	 * In the unlikely event that ndblk.prune_sb is set to another
+	 * super_block, the waiting will last the complete pruning operation
+	 * which shouldn't be that long either.
+	 */
+	spin_unlock_wait(&ndblk.prune_lock);
+}
+
 /**
  * enum d_walk_ret - action to talke during tree walk
  * @D_WALK_CONTINUE:	contrinue walk
@@ -1755,6 +1789,7 @@ void shrink_dcache_for_umount(struct super_block *sb)
 
 	WARN(down_read_trylock(&sb->s_umount), "s_umount should've been locked");
 
+	clear_prune_sb_for_umount(sb);
 	dentry = sb->s_root;
 	sb->s_root = NULL;
 	do_one_tree(dentry);
@@ -3857,6 +3892,7 @@ static void __init neg_dentry_init(void)
 	unsigned long cnt;
 
 	raw_spin_lock_init(&ndblk.nfree_lock);
+	spin_lock_init(&ndblk.prune_lock);
 
 	/* 20% in global pool & 80% in percpu free */
 	ndblk.nfree = neg_dentry_nfree_init
-- 
1.8.3.1