autofs4: d_manage() should return -EISDIR when appropriate in rcu-walk mode.

If rcu-walk mode we don't *have* to return -EISDIR for non-mount-traps
as we will simply drop into REF-walk and handling DCACHE_NEED_AUTOMOUNT
dentrys the slow way.  But it is better if we do when possible.

In 'oz_mode', use the same condition as ref-walk: if not a mountpoint,
then it must be -EISDIR.

In regular mode there are most tests needed.  Most of them can be
performed without taking any spinlocks.  If we find a directory that
isn't obviously empty, and isn't mounted on, we need to call
'simple_empty()' which does take a spinlock.  If this turned out to hurt
performance, some other approach could be found to signal when a
directory is known to be empty.

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Ian Kent <raven@themaw.net>
Tested-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 20 insertions, 6 deletions

diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index 2296c8301b66..d76d083f2f06 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -433,8 +433,6 @@ static int autofs4_d_manage(struct dentry *dentry, bool rcu_walk)
 
         /* The daemon never waits. */
         if (autofs4_oz_mode(sbi)) {
-                if (rcu_walk)
-                        return 0;
                 if (!d_mountpoint(dentry))
                         return -EISDIR;
                 return 0;
@@ -452,12 +450,28 @@ static int autofs4_d_manage(struct dentry *dentry, bool rcu_walk)
         if (status)
                 return status;
 
-        if (rcu_walk)
-                /* it is always safe to return 0 as the worst that
-                 * will happen is we retry in REF-walk mode.
-                 * Better than always taking a lock.
+        if (rcu_walk) {
+                /* We don't need fs_lock in rcu_walk mode,
+                 * just testing 'AUTOFS_INFO_NO_RCU' is enough.
+                 * simple_empty() takes a spinlock, so leave it
+                 * to last.
+                 * We only return -EISDIR when certain this isn't
+                 * a mount-trap.
                  */
+                struct inode *inode;
+                if (ino->flags & (AUTOFS_INF_EXPIRING | AUTOFS_INF_NO_RCU))
+                        return 0;
+                if (d_mountpoint(dentry))
+                        return 0;
+                inode = ACCESS_ONCE(dentry->d_inode);
+                if (inode && S_ISLNK(inode->i_mode))
+                        return -EISDIR;
+                if (list_empty(&dentry->d_subdirs))
+                        return 0;
+                if (!simple_empty(dentry))
+                        return -EISDIR;
                 return 0;
+        }
 
         spin_lock(&sbi->fs_lock);
         /*