lockless get_write_access/deny_write_access

new helpers: atomic_inc_unless_negative()/atomic_dec_unless_positive()
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -341,52 +341,6 @@ static inline int exec_permission(struct inode *inode, unsigned int flags)
 	return security_inode_exec_permission(inode, flags);
 }
 
-/*
- * get_write_access() gets write permission for a file.
- * put_write_access() releases this write permission.
- * This is used for regular files.
- * We cannot support write (and maybe mmap read-write shared) accesses and
- * MAP_DENYWRITE mmappings simultaneously. The i_writecount field of an inode
- * can have the following values:
- * 0: no writers, no VM_DENYWRITE mappings
- * < 0: (-i_writecount) vm_area_structs with VM_DENYWRITE set exist
- * > 0: (i_writecount) users are writing to the file.
- *
- * Normally we operate on that counter with atomic_{inc,dec} and it's safe
- * except for the cases where we don't hold i_writecount yet. Then we need to
- * use {get,deny}_write_access() - these functions check the sign and refuse
- * to do the change if sign is wrong. Exclusion between them is provided by
- * the inode->i_lock spinlock.
- */
-
-int get_write_access(struct inode * inode)
-{
-	spin_lock(&inode->i_lock);
-	if (atomic_read(&inode->i_writecount) < 0) {
-		spin_unlock(&inode->i_lock);
-		return -ETXTBSY;
-	}
-	atomic_inc(&inode->i_writecount);
-	spin_unlock(&inode->i_lock);
-
-	return 0;
-}
-
-int deny_write_access(struct file * file)
-{
-	struct inode *inode = file->f_path.dentry->d_inode;
-
-	spin_lock(&inode->i_lock);
-	if (atomic_read(&inode->i_writecount) > 0) {
-		spin_unlock(&inode->i_lock);
-		return -ETXTBSY;
-	}
-	atomic_dec(&inode->i_writecount);
-	spin_unlock(&inode->i_lock);
-
-	return 0;
-}
-
 /**
  * path_get - get a reference to a path
  * @path: path to get the reference to

include/linux/atomic.h

@@ -34,6 +34,32 @@ static inline int atomic_inc_not_zero_hint(atomic_t *v, int hint)
 }
 #endif
 
+#ifndef atomic_inc_unless_negative
+static inline int atomic_inc_unless_negative(atomic_t *p)
+{
+	int v, v1;
+	for (v = 0; v >= 0; v = v1) {
+		v1 = atomic_cmpxchg(p, v, v + 1);
+		if (likely(v1 == v))
+			return 1;
+	}
+	return 0;
+}
+#endif
+
+#ifndef atomic_dec_unless_positive
+static inline int atomic_dec_unless_positive(atomic_t *p)
+{
+	int v, v1;
+	for (v = 0; v <= 0; v = v1) {
+		v1 = atomic_cmpxchg(p, v, v - 1);
+		if (likely(v1 == v))
+			return 1;
+	}
+	return 0;
+}
+#endif
+
 #ifndef CONFIG_ARCH_HAS_ATOMIC_OR
 static inline void atomic_or(int i, atomic_t *v)
 {

include/linux/fs.h

@@ -392,8 +392,8 @@ struct inodes_stat_t {
 #include <linux/semaphore.h>
 #include <linux/fiemap.h>
 #include <linux/rculist_bl.h>
+#include <linux/atomic.h>
 
-#include <asm/atomic.h>
 #include <asm/byteorder.h>
 
 struct export_operations;
@@ -2195,8 +2195,31 @@ static inline bool execute_ok(struct inode *inode)
 	return (inode->i_mode & S_IXUGO) || S_ISDIR(inode->i_mode);
 }
 
-extern int get_write_access(struct inode *);
-extern int deny_write_access(struct file *);
+/*
+ * get_write_access() gets write permission for a file.
+ * put_write_access() releases this write permission.
+ * This is used for regular files.
+ * We cannot support write (and maybe mmap read-write shared) accesses and
+ * MAP_DENYWRITE mmappings simultaneously. The i_writecount field of an inode
+ * can have the following values:
+ * 0: no writers, no VM_DENYWRITE mappings
+ * < 0: (-i_writecount) vm_area_structs with VM_DENYWRITE set exist
+ * > 0: (i_writecount) users are writing to the file.
+ *
+ * Normally we operate on that counter with atomic_{inc,dec} and it's safe
+ * except for the cases where we don't hold i_writecount yet. Then we need to
+ * use {get,deny}_write_access() - these functions check the sign and refuse
+ * to do the change if sign is wrong.
+ */
+static inline int get_write_access(struct inode *inode)
+{
+	return atomic_inc_unless_negative(&inode->i_writecount) ? 0 : -ETXTBSY;
+}
+static inline int deny_write_access(struct file *file)
+{
+	struct inode *inode = file->f_path.dentry->d_inode;
+	return atomic_dec_unless_positive(&inode->i_writecount) ? 0 : -ETXTBSY;
+}
 static inline void put_write_access(struct inode * inode)
 {
 	atomic_dec(&inode->i_writecount);