[RFC] get_write_access()/deny_write_access() without inode->i_lock

[Al Viro <viro@ZenIV.linux.org.uk>]

	I'm seriously tempted to throw away i_lock uses in
{get,deny}_write_access(), as in the patch below.  The question is, how
badly will it suck on various architectures?  I'd expect it to be not
worse than the current version, but...
	BTW, I wonder if we need barriers in {put,allow}_write_access (in
either version).

	Related question: would it make sense to turn that into
atomic_inc_unless_negative/atomic_dec_unless_positive?  I don't
remember any code doing that kind of stuff - no idea if there are
any potential users for that.

diff --git a/fs/namei.c b/fs/namei.c
index 26bef77..7dffe2e 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -341,52 +341,6 @@ ok:
 	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
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7302e44..ab89aa3 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2194,8 +2194,43 @@ 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)
+{
+	int v, v1;
+	for (v = atomic_read(&inode->i_writecount); v >= 0; v = v1) {
+		v1 = atomic_cmpxchg(&inode->i_writecount, v, v + 1);
+		if (likely(v1 == v))
+			return 0;
+	}
+	return -ETXTBSY;
+}
+static inline int deny_write_access(struct file *file)
+{
+	struct inode *inode = file->f_path.dentry->d_inode;
+	int v, v1;
+	for (v = atomic_read(&inode->i_writecount); v <= 0; v = v1) {
+		v1 = atomic_cmpxchg(&inode->i_writecount, v, v + 1);
+		if (likely(v1 == v))
+			return 0;
+	}
+	return -ETXTBSY;
+}
 static inline void put_write_access(struct inode * inode)
 {
 	atomic_dec(&inode->i_writecount);