untangle do_lookup()

That thing has devolved into rats nest of gotos; sane use of unlikely()
gets rid of that horror and gives much more readable structure:
	* make a fast attempt to find a dentry; false negatives are OK.
In RCU mode if everything went fine, we are done, otherwise just drop
out of RCU.  If we'd done (RCU) ->d_revalidate() and it had not refused
outright (i.e. didn't give us -ECHILD), remember its result.
	* now we are not in RCU mode and hopefully have a dentry.  If we
do not, lock parent, do full d_lookup() and if that has not found anything,
allocate and call ->lookup().  If we'd done that ->lookup(), remember that
dentry is good and we don't need to revalidate it.
	* now we have a dentry.  If it has ->d_revalidate() and we can't
skip it, call it.
	* hopefully dentry is good; if not, either fail (in case of error)
or try to invalidate it.  If d_invalidate() has succeeded, drop it and
retry everything as if original attempt had not found a dentry.
	* now we can finish it up - deal with mountpoint crossing and
automount.

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

1 files changed, 56 insertions, 85 deletions

diff --git a/fs/namei.c b/fs/namei.c
index ca9a06a65704..0bebd13e5cb7 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -589,29 +589,6 @@ do_revalidate(struct dentry *dentry, struct nameidata *nd)
         return dentry;
 }
 
-static inline struct dentry *
-do_revalidate_rcu(struct dentry *dentry, struct nameidata *nd)
-{
-        int status = d_revalidate(dentry, nd);
-        if (likely(status > 0))
-                return dentry;
-        if (status == -ECHILD) {
-                if (nameidata_dentry_drop_rcu(nd, dentry))
-                        return ERR_PTR(-ECHILD);
-                return do_revalidate(dentry, nd);
-        }
-        if (status < 0)
-                return ERR_PTR(status);
-        /* Don't d_invalidate in rcu-walk mode */
-        if (nameidata_dentry_drop_rcu(nd, dentry))
-                return ERR_PTR(-ECHILD);
-        if (!d_invalidate(dentry)) {
-                dput(dentry);
-                dentry = NULL;
-        }
-        return dentry;
-}
-
 /*
  * handle_reval_path - force revalidation of a dentry
  *
@@ -1213,7 +1190,8 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
 {
         struct vfsmount *mnt = nd->path.mnt;
         struct dentry *dentry, *parent = nd->path.dentry;
-        struct inode *dir;
+        int need_reval = 1;
+        int status = 1;
         int err;
 
         /*
@@ -1223,48 +1201,74 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
          */
         if (nd->flags & LOOKUP_RCU) {
                 unsigned seq;
-
                 *inode = nd->inode;
                 dentry = __d_lookup_rcu(parent, name, &seq, inode);
-                if (!dentry) {
-                        if (nameidata_drop_rcu(nd))
-                                return -ECHILD;
-                        goto need_lookup;
-                }
+                if (!dentry)
+                        goto unlazy;
+
                 /* Memory barrier in read_seqcount_begin of child is enough */
                 if (__read_seqcount_retry(&parent->d_seq, nd->seq))
                         return -ECHILD;
-
                 nd->seq = seq;
+
                 if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE)) {
-                        dentry = do_revalidate_rcu(dentry, nd);
-                        if (!dentry)
-                                goto need_lookup;
-                        if (IS_ERR(dentry))
-                                goto fail;
-                        if (!(nd->flags & LOOKUP_RCU))
-                                goto done;
+                        status = d_revalidate(dentry, nd);
+                        if (unlikely(status <= 0)) {
+                                if (status != -ECHILD)
+                                        need_reval = 0;
+                                goto unlazy;
+                        }
                 }
                 path->mnt = mnt;
                 path->dentry = dentry;
                 if (likely(__follow_mount_rcu(nd, path, inode, false)))
                         return 0;
-                if (nameidata_drop_rcu(nd))
-                        return -ECHILD;
-                /* fallthru */
+unlazy:
+                if (dentry) {
+                        if (nameidata_dentry_drop_rcu(nd, dentry))
+                                return -ECHILD;
+                } else {
+                        if (nameidata_drop_rcu(nd))
+                                return -ECHILD;
+                }
+        } else {
+                dentry = __d_lookup(parent, name);
         }
-        dentry = __d_lookup(parent, name);
-        if (!dentry)
-                goto need_lookup;
-found:
-        if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE)) {
-                dentry = do_revalidate(dentry, nd);
-                if (!dentry)
-                        goto need_lookup;
-                if (IS_ERR(dentry))
-                        goto fail;
+
+retry:
+        if (unlikely(!dentry)) {
+                struct inode *dir = parent->d_inode;
+                BUG_ON(nd->inode != dir);
+
+                mutex_lock(&dir->i_mutex);
+                dentry = d_lookup(parent, name);
+                if (likely(!dentry)) {
+                        dentry = d_alloc_and_lookup(parent, name, nd);
+                        if (IS_ERR(dentry)) {
+                                mutex_unlock(&dir->i_mutex);
+                                return PTR_ERR(dentry);
+                        }
+                        /* known good */
+                        need_reval = 0;
+                        status = 1;
+                }
+                mutex_unlock(&dir->i_mutex);
+        }
+        if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE) && need_reval)
+                status = d_revalidate(dentry, nd);
+        if (unlikely(status <= 0)) {
+                if (status < 0) {
+                        dput(dentry);
+                        return status;
+                }
+                if (!d_invalidate(dentry)) {
+                        dput(dentry);
+                        dentry = NULL;
+                        need_reval = 1;
+                        goto retry;
+                }
         }
-done:
+
         path->mnt = mnt;
         path->dentry = dentry;
         err = follow_managed(path, nd->flags);
@@ -1274,39 +1278,6 @@ done:
         }
         *inode = path->dentry->d_inode;
         return 0;
-
-need_lookup:
-        dir = parent->d_inode;
-        BUG_ON(nd->inode != dir);
-
-        mutex_lock(&dir->i_mutex);
-        /*
-         * First re-do the cached lookup just in case it was created
-         * while we waited for the directory semaphore, or the first
-         * lookup failed due to an unrelated rename.
-         *
-         * This could use version numbering or similar to avoid unnecessary
-         * cache lookups, but then we'd have to do the first lookup in the
-         * non-racy way. However in the common case here, everything should
-         * be hot in cache, so would it be a big win?
-         */
-        dentry = d_lookup(parent, name);
-        if (likely(!dentry)) {
-                dentry = d_alloc_and_lookup(parent, name, nd);
-                mutex_unlock(&dir->i_mutex);
-                if (IS_ERR(dentry))
-                        goto fail;
-                goto done;
-        }
-        /*
-         * Uhhuh! Nasty case: the cache was re-populated while
-         * we waited on the semaphore. Need to revalidate.
-         */
-        mutex_unlock(&dir->i_mutex);
-        goto found;
-
-fail:
-        return PTR_ERR(dentry);
 }
 
 static inline int may_lookup(struct nameidata *nd)