untangle do_lookup()
Al Viro [Fri, 11 Mar 2011 09:44:53 +0000 (04:44 -0500)]
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>

fs/namei.c

index ca9a06a..0bebd13 100644 (file)
@@ -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)