fs: Use rename lock and RCU for multi-step operations

The remaining usages for dcache_lock is to allow atomic, multi-step read-side
operations over the directory tree by excluding modifications to the tree.
Also, to walk in the leaf->root direction in the tree where we don't have
a natural d_lock ordering.

This could be accomplished by taking every d_lock, but this would mean a
huge number of locks and actually gets very tricky.

Solve this instead by using the rename seqlock for multi-step read-side
operations, retry in case of a rename so we don't walk up the wrong parent.
Concurrent dentry insertions are not serialised against.  Concurrent deletes
are tricky when walking up the directory: our parent might have been deleted
when dropping locks so also need to check and retry for that.

We can also use the rename lock in cases where livelock is a worry (and it
is introduced in subsequent patch).

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

1 files changed, 15 insertions, 3 deletions

diff --git a/fs/autofs4/waitq.c b/fs/autofs4/waitq.c
index 2341375386f8..4be8f778a418 100644
--- a/fs/autofs4/waitq.c
+++ b/fs/autofs4/waitq.c
@@ -186,16 +186,25 @@ static int autofs4_getpath(struct autofs_sb_info *sbi,
 {
         struct dentry *root = sbi->sb->s_root;
         struct dentry *tmp;
-        char *buf = *name;
+        char *buf;
         char *p;
-        int len = 0;
-
+        int len;
+        unsigned seq;
+
+rename_retry:
+        buf = *name;
+        len = 0;
+        seq = read_seqbegin(&rename_lock);
+        rcu_read_lock();
         spin_lock(&dcache_lock);
         for (tmp = dentry ; tmp != root ; tmp = tmp->d_parent)
                 len += tmp->d_name.len + 1;
 
         if (!len || --len > NAME_MAX) {
                 spin_unlock(&dcache_lock);
+                rcu_read_unlock();
+                if (read_seqretry(&rename_lock, seq))
+                        goto rename_retry;
                 return 0;
         }
 
@@ -209,6 +218,9 @@ static int autofs4_getpath(struct autofs_sb_info *sbi,
                 strncpy(p, tmp->d_name.name, tmp->d_name.len);
         }
         spin_unlock(&dcache_lock);
+        rcu_read_unlock();
+        if (read_seqretry(&rename_lock, seq))
+                goto rename_retry;
 
         return len;
 }