get rid of trylock loop in locking dentries on shrink list
In case of trylock failure don't re-add to the list - drop the locks
and carefully get them in the right order. For shrink_dentry_list(),
somebody having grabbed a reference to dentry means that we can
kick it off-list, so if we find dentry being modified under us we
don't need to play silly buggers with retries anyway - off the list
it is.
The locking logics taken out into a helper of its own; lock_parent()
is no longer used for dentries that can be killed under us.
[fix from Eric Biggers folded]
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/dcache.c b/fs/dcache.c
index 1684b6b..af85014 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -974,56 +974,86 @@
}
EXPORT_SYMBOL(d_prune_aliases);
+/*
+ * Lock a dentry from shrink list.
+ * Note that dentry is *not* protected from concurrent dentry_kill(),
+ * d_delete(), etc. It is protected from freeing (by the fact of
+ * being on a shrink list), but everything else is fair game.
+ * Return false if dentry has been disrupted or grabbed, leaving
+ * the caller to kick it off-list. Otherwise, return true and have
+ * that dentry's inode and parent both locked.
+ */
+static bool shrink_lock_dentry(struct dentry *dentry)
+{
+ struct inode *inode;
+ struct dentry *parent;
+
+ if (dentry->d_lockref.count)
+ return false;
+
+ inode = dentry->d_inode;
+ if (inode && unlikely(!spin_trylock(&inode->i_lock))) {
+ rcu_read_lock(); /* to protect inode */
+ spin_unlock(&dentry->d_lock);
+ spin_lock(&inode->i_lock);
+ spin_lock(&dentry->d_lock);
+ if (unlikely(dentry->d_lockref.count))
+ goto out;
+ /* changed inode means that somebody had grabbed it */
+ if (unlikely(inode != dentry->d_inode))
+ goto out;
+ rcu_read_unlock();
+ }
+
+ parent = dentry->d_parent;
+ if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock)))
+ return true;
+
+ rcu_read_lock(); /* to protect parent */
+ spin_unlock(&dentry->d_lock);
+ parent = READ_ONCE(dentry->d_parent);
+ spin_lock(&parent->d_lock);
+ if (unlikely(parent != dentry->d_parent)) {
+ spin_unlock(&parent->d_lock);
+ spin_lock(&dentry->d_lock);
+ goto out;
+ }
+ spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
+ if (likely(!dentry->d_lockref.count)) {
+ rcu_read_unlock();
+ return true;
+ }
+ spin_unlock(&parent->d_lock);
+out:
+ if (inode)
+ spin_unlock(&inode->i_lock);
+ rcu_read_unlock();
+ return false;
+}
+
static void shrink_dentry_list(struct list_head *list)
{
- struct dentry *dentry, *parent;
-
while (!list_empty(list)) {
+ struct dentry *dentry, *parent;
struct inode *inode;
+
dentry = list_entry(list->prev, struct dentry, d_lru);
spin_lock(&dentry->d_lock);
- parent = lock_parent(dentry);
-
- /*
- * The dispose list is isolated and dentries are not accounted
- * to the LRU here, so we can simply remove it from the list
- * here regardless of whether it is referenced or not.
- */
- d_shrink_del(dentry);
-
- /*
- * We found an inuse dentry which was not removed from
- * the LRU because of laziness during lookup. Do not free it.
- */
- if (dentry->d_lockref.count > 0) {
+ if (!shrink_lock_dentry(dentry)) {
+ bool can_free = false;
+ d_shrink_del(dentry);
+ if (dentry->d_lockref.count < 0)
+ can_free = dentry->d_flags & DCACHE_MAY_FREE;
spin_unlock(&dentry->d_lock);
- if (parent)
- spin_unlock(&parent->d_lock);
- continue;
- }
-
-
- if (unlikely(dentry->d_flags & DCACHE_DENTRY_KILLED)) {
- bool can_free = dentry->d_flags & DCACHE_MAY_FREE;
- spin_unlock(&dentry->d_lock);
- if (parent)
- spin_unlock(&parent->d_lock);
if (can_free)
dentry_free(dentry);
continue;
}
-
- inode = dentry->d_inode;
- if (inode && unlikely(!spin_trylock(&inode->i_lock))) {
- d_shrink_add(dentry, list);
- spin_unlock(&dentry->d_lock);
- if (parent)
- spin_unlock(&parent->d_lock);
- continue;
- }
-
+ d_shrink_del(dentry);
+ parent = dentry->d_parent;
__dentry_kill(dentry);
-
+ if (parent == dentry)
+ continue;
/*
* We need to prune ancestors too. This is necessary to prevent
* quadratic behavior of shrink_dcache_parent(), but is also