lockless next_positive()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 27 insertions, 5 deletions

diff --git a/fs/libfs.c b/fs/libfs.c
index b05b74ae3f16..74dc8b9e7f53 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -89,31 +89,53 @@ static struct dentry *next_positive(struct dentry *parent,
                                     struct list_head *from,
                                     int count)
 {
-        struct dentry *res = NULL;
+        unsigned *seq = &parent->d_inode->i_dir_seq, n;
+        struct dentry *res;
         struct list_head *p;
+        bool skipped;
+        int i;
 
-        spin_lock(&parent->d_lock);
+retry:
+        i = count;
+        skipped = false;
+        n = smp_load_acquire(seq) & ~1;
+        res = NULL;
+        rcu_read_lock();
         for (p = from->next; p != &parent->d_subdirs; p = p->next) {
                 struct dentry *d = list_entry(p, struct dentry, d_child);
-                if (simple_positive(d) && !--count) {
+                if (!simple_positive(d)) {
+                        skipped = true;
+                } else if (!--i) {
                         res = d;
                         break;
                 }
         }
-        spin_unlock(&parent->d_lock);
+        rcu_read_unlock();
+        if (skipped) {
+                smp_rmb();
+                if (unlikely(*seq != n))
+                        goto retry;
+        }
         return res;
 }
 
 static void move_cursor(struct dentry *cursor, struct list_head *after)
 {
         struct dentry *parent = cursor->d_parent;
-
+        unsigned n, *seq = &parent->d_inode->i_dir_seq;
         spin_lock(&parent->d_lock);
+        for (;;) {
+                n = *seq;
+                if (!(n & 1) && cmpxchg(seq, n, n + 1) == n)
+                        break;
+                cpu_relax();
+        }
         __list_del(cursor->d_child.prev, cursor->d_child.next);
         if (after)
                 list_add(&cursor->d_child, after);
         else
                 list_add_tail(&cursor->d_child, &parent->d_subdirs);
+        smp_store_release(seq, n + 2);
         spin_unlock(&parent->d_lock);
 }