autofs4: don't take spinlock when not needed in autofs4_lookup_expiring

If the expiring_list is empty, we can avoid a costly spinlock in the
rcu-walk path through autofs4_d_manage (once the rest of the path
becomes rcu-walk friendly).

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

diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index cc87c1a..fb202ca 100644 (file)
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -166,8 +166,10 @@ static struct dentry *autofs4_lookup_active(struct dentry *dentry)
        const unsigned char *str = name->name;
        struct list_head *p, *head;
 
-       spin_lock(&sbi->lookup_lock);
        head = &sbi->active_list;
+       if (list_empty(head))
+               return NULL;
+       spin_lock(&sbi->lookup_lock);
        list_for_each(p, head) {
                struct autofs_info *ino;
                struct dentry *active;
@@ -218,8 +220,10 @@ static struct dentry *autofs4_lookup_expiring(struct dentry *dentry)
        const unsigned char *str = name->name;
        struct list_head *p, *head;
 
-       spin_lock(&sbi->lookup_lock);
        head = &sbi->expiring_list;
+       if (list_empty(head))
+               return NULL;
+       spin_lock(&sbi->lookup_lock);
        list_for_each(p, head) {
                struct autofs_info *ino;
                struct dentry *expiring;