RCUify freeing acls, let check_acl() go ahead in RCU mode if acl is cached

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

fs/namei.c

@@ -179,19 +179,14 @@ static int check_acl(struct inode *inode, int mask)
 #ifdef CONFIG_FS_POSIX_ACL
 	struct posix_acl *acl;
 
-	/*
-	 * Under RCU walk, we cannot even do a "get_cached_acl()",
-	 * because that involves locking and getting a refcount on
-	 * a cached ACL.
-	 *
-	 * So the only case we handle during RCU walking is the
-	 * case of a cached "no ACL at all", which needs no locks
-	 * or refcounts.
-	 */
 	if (mask & MAY_NOT_BLOCK) {
-	        if (negative_cached_acl(inode, ACL_TYPE_ACCESS))
+		acl = get_cached_acl_rcu(inode, ACL_TYPE_ACCESS);
+	        if (!acl)
 	                return -EAGAIN;
+		/* no ->get_acl() calls in RCU mode... */
+		if (acl == ACL_NOT_CACHED)
 			return -ECHILD;
+	        return posix_acl_permission(inode, acl, mask);
 	}
 
 	acl = get_cached_acl(inode, ACL_TYPE_ACCESS);

include/linux/posix_acl.h

@@ -9,6 +9,7 @@
 #define __LINUX_POSIX_ACL_H
 
 #include <linux/slab.h>
+#include <linux/rcupdate.h>
 
 #define ACL_UNDEFINED_ID	(-1)
 
@@ -38,7 +39,10 @@ struct posix_acl_entry {
 };
 
 struct posix_acl {
+	union {
 		atomic_t		a_refcount;
+		struct rcu_head		a_rcu;
+	};
 	unsigned int		a_count;
 	struct posix_acl_entry	a_entries[0];
 };
@@ -65,7 +69,7 @@ static inline void
 posix_acl_release(struct posix_acl *acl)
 {
 	if (acl && atomic_dec_and_test(&acl->a_refcount))
-		kfree(acl);
+		kfree_rcu(acl, a_rcu);
 }
 
 
@@ -110,13 +114,9 @@ static inline struct posix_acl *get_cached_acl(struct inode *inode, int type)
 	return acl;
 }
 
-static inline int negative_cached_acl(struct inode *inode, int type)
+static inline struct posix_acl *get_cached_acl_rcu(struct inode *inode, int type)
 {
-	struct posix_acl **p = acl_by_type(inode, type);
-	struct posix_acl *acl = ACCESS_ONCE(*p);
-	if (acl)
-		return 0;
-	return 1;
+	return rcu_dereference(*acl_by_type(inode, type));
 }
 
 static inline void set_cached_acl(struct inode *inode,
@@ -127,7 +127,7 @@ static inline void set_cached_acl(struct inode *inode,
 	struct posix_acl *old;
 	spin_lock(&inode->i_lock);
 	old = *p;
-	*p = posix_dup_acl(acl);
+	rcu_assign_pointer(*p, posix_acl_dup(acl));
 	spin_unlock(&inode->i_lock);
 	if (old != ACL_NOT_CACHED)
 		posix_acl_release(old);