don't bother with {get,put}_write_access() on non-regular files

it's pointless and actually leads to wrong behaviour in at least one
moderately convoluted case (pipe(), close one end, try to get to
another via /proc/*/fd and run into ETXTBUSY).

Cc: [e-mail]
Signed-off-by: Al Viro <[e-mail]>

fs/file_table.c

···
 	struct dentry *dentry = file->f_path.dentry;
 	struct inode *inode = dentry->d_inode;
 
-	put_write_access(inode);
-
 	if (special_file(inode->i_mode))
 		return;
+
+	put_write_access(inode);
 	if (file_check_writeable(file) != 0)
 		return;
 	__mnt_drop_write(mnt);

fs/open.c

···
 static inline int __get_file_write_access(struct inode *inode,
 					  struct vfsmount *mnt)
 {
-	int error;
-	error = get_write_access(inode);
+	int error = get_write_access(inode);
 	if (error)
 		return error;
-	/*
-	 * Do not take mount writer counts on
-	 * special files since no writes to
-	 * the mount itself will occur.
-	 */
-	if (!special_file(inode->i_mode)) {
-		/*
-		 * Balanced in __fput()
-		 */
-		error = __mnt_want_write(mnt);
-		if (error)
-			put_write_access(inode);
-	}
+	error = __mnt_want_write(mnt);
+	if (error)
+		put_write_access(inode);
 	return error;
 }
 
···
 
 	path_get(&f->f_path);
 	inode = f->f_inode = f->f_path.dentry->d_inode;
-	if (f->f_mode & FMODE_WRITE) {
+	if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
 		error = __get_file_write_access(inode, f->f_path.mnt);
 		if (error)
 			goto cleanup_file;
-		if (!special_file(inode->i_mode))
-			file_take_write(f);
+		file_take_write(f);
 	}
 
 	f->f_mapping = inode->i_mapping;
···
 cleanup_all:
 	fops_put(f->f_op);
 	if (f->f_mode & FMODE_WRITE) {
-		put_write_access(inode);
 		if (!special_file(inode->i_mode)) {
 			/*
 			 * We don't consider this a real
···
 			 * because it all happenend right
 			 * here, so just reset the state.
 			 */
+			put_write_access(inode);
 			file_reset_write(f);
 			__mnt_drop_write(f->f_path.mnt);
 		}