list_lru: remove special case function list_lru_dispose_all.

The list_lru implementation has one function, list_lru_dispose_all, with
only one user (the dentry code).  At first, such function appears to make
sense because we are really not interested in the result of isolating each
dentry separately - all of them are going away anyway.  However, it's
implementation is buggy in the following way:

When we call list_lru_dispose_all in fs/dcache.c, we scan all dentries
marking them with DCACHE_SHRINK_LIST.  However, this is done without the
nlru->lock taken.  The imediate result of that is that someone else may
add or remove the dentry from the LRU at the same time.  When list_lru_del
happens in that scenario we will see an element that is not yet marked
with DCACHE_SHRINK_LIST (even though it will be in the future) and
obviously remove it from an lru where the element no longer is.  Since
list_lru_dispose_all will in effect count down nlru's nr_items and
list_lru_del will do the same, this will lead to an imbalance.

The solution for this would not be so simple: we can obviously just keep
the lru_lock taken, but then we have no guarantees that we will be able to
acquire the dentry lock (dentry->d_lock).  To properly solve this, we need
a communication mechanism between the lru and dentry code, so they can
coordinate this with each other.

Such mechanism already exists in the form of the list_lru_walk_cb
callback.  So it is possible to construct a dcache-side prune function
that does the right thing only by calling list_lru_walk in a loop until no
more dentries are available.

With only one user, plus the fact that a sane solution for the problem
would involve boucing between dcache and list_lru anyway, I see little
justification to keep the special case list_lru_dispose_all in tree.

Signed-off-by: Glauber Costa <glommer@openvz.org>
Cc: Michal Hocko <mhocko@suse.cz>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 29 insertions, 20 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 2275bd416736..21f36b4d95cc 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -910,27 +910,29 @@ long prune_dcache_sb(struct super_block *sb, unsigned long nr_to_scan)
 	return freed;
 }
 
-/*
- * Mark all the dentries as on being the dispose list so we don't think they are
- * still on the LRU if we try to kill them from ascending the parent chain in
- * try_prune_one_dentry() rather than directly from the dispose list.
- */
-static void
-shrink_dcache_list(
-	struct list_head *dispose)
+static enum lru_status dentry_lru_isolate_shrink(struct list_head *item,
+						spinlock_t *lru_lock, void *arg)
 {
-	struct dentry *dentry;
+	struct list_head *freeable = arg;
+	struct dentry	*dentry = container_of(item, struct dentry, d_lru);
 
-	rcu_read_lock();
-	list_for_each_entry_rcu(dentry, dispose, d_lru) {
-		spin_lock(&dentry->d_lock);
-		dentry->d_flags |= DCACHE_SHRINK_LIST;
-		spin_unlock(&dentry->d_lock);
-	}
-	rcu_read_unlock();
-	shrink_dentry_list(dispose);
+	/*
+	 * we are inverting the lru lock/dentry->d_lock here,
+	 * so use a trylock. If we fail to get the lock, just skip
+	 * it
+	 */
+	if (!spin_trylock(&dentry->d_lock))
+		return LRU_SKIP;
+
+	dentry->d_flags |= DCACHE_SHRINK_LIST;
+	list_move_tail(&dentry->d_lru, freeable);
+	this_cpu_dec(nr_dentry_unused);
+	spin_unlock(&dentry->d_lock);
+
+	return LRU_REMOVED;
 }
 
+
 /**
  * shrink_dcache_sb - shrink dcache for a superblock
  * @sb: superblock
@@ -940,10 +942,17 @@ shrink_dcache_list(
  */
 void shrink_dcache_sb(struct super_block *sb)
 {
-	long disposed;
+	long freed;
 
-	disposed = list_lru_dispose_all(&sb->s_dentry_lru, shrink_dcache_list);
-	this_cpu_sub(nr_dentry_unused, disposed);
+	do {
+		LIST_HEAD(dispose);
+
+		freed = list_lru_walk(&sb->s_dentry_lru,
+			dentry_lru_isolate_shrink, &dispose, UINT_MAX);
+
+		this_cpu_sub(nr_dentry_unused, freed);
+		shrink_dentry_list(&dispose);
+	} while (freed > 0);
 }
 EXPORT_SYMBOL(shrink_dcache_sb);