fs/9p: Fix race in initializing writeback fid

When two process open the same file we can end up with both of them
allocating the writeback_fid. Add a new mutex which can be used
for synchronizing v9fs_inode member values.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

fs/9p/v9fs.h

@@ -130,6 +130,7 @@ struct v9fs_inode {
 #endif
 	unsigned int cache_validity;
 	struct p9_fid *writeback_fid;
+	struct mutex v_mutex;
 	struct inode vfs_inode;
 };
 

fs/9p/vfs_file.c

@@ -90,6 +90,7 @@ int v9fs_file_open(struct inode *inode, struct file *file)
 	}
 
 	file->private_data = fid;
+	mutex_lock(&v9inode->v_mutex);
 	if (v9ses->cache && !v9inode->writeback_fid) {
 		/*
 		 * clone a fid and add it to writeback_fid
@@ -101,10 +102,12 @@ int v9fs_file_open(struct inode *inode, struct file *file)
 		fid = v9fs_writeback_fid(file->f_path.dentry);
 		if (IS_ERR(fid)) {
 			err = PTR_ERR(fid);
+			mutex_unlock(&v9inode->v_mutex);
 			goto out_error;
 		}
 		v9inode->writeback_fid = (void *) fid;
 	}
+	mutex_unlock(&v9inode->v_mutex);
 #ifdef CONFIG_9P_FSCACHE
 	if (v9ses->cache)
 		v9fs_cache_inode_set_cookie(inode, file);

fs/9p/vfs_inode.c

@@ -221,6 +221,7 @@ struct inode *v9fs_alloc_inode(struct super_block *sb)
 #endif
 	v9inode->writeback_fid = NULL;
 	v9inode->cache_validity = 0;
+	mutex_init(&v9inode->v_mutex);
 	return &v9inode->vfs_inode;
 }
 
@@ -650,6 +651,7 @@ v9fs_vfs_create(struct inode *dir, struct dentry *dentry, int mode,
 	/* if we are opening a file, assign the open fid to the file */
 	if (nd && nd->flags & LOOKUP_OPEN) {
 		v9inode = V9FS_I(dentry->d_inode);
+		mutex_lock(&v9inode->v_mutex);
 		if (v9ses->cache && !v9inode->writeback_fid) {
 			/*
 			 * clone a fid and add it to writeback_fid
@@ -661,10 +663,12 @@ v9fs_vfs_create(struct inode *dir, struct dentry *dentry, int mode,
 			inode_fid = v9fs_writeback_fid(dentry);
 			if (IS_ERR(inode_fid)) {
 				err = PTR_ERR(inode_fid);
+				mutex_unlock(&v9inode->v_mutex);
 				goto error;
 			}
 			v9inode->writeback_fid = (void *) inode_fid;
 		}
+		mutex_unlock(&v9inode->v_mutex);
 		filp = lookup_instantiate_filp(nd, dentry, generic_file_open);
 		if (IS_ERR(filp)) {
 			err = PTR_ERR(filp);

fs/9p/vfs_inode_dotl.c

@@ -245,6 +245,7 @@ v9fs_vfs_create_dotl(struct inode *dir, struct dentry *dentry, int omode,
 	v9fs_set_create_acl(dentry, dacl, pacl);
 
 	v9inode = V9FS_I(inode);
+	mutex_lock(&v9inode->v_mutex);
 	if (v9ses->cache && !v9inode->writeback_fid) {
 		/*
 		 * clone a fid and add it to writeback_fid
@@ -256,10 +257,12 @@ v9fs_vfs_create_dotl(struct inode *dir, struct dentry *dentry, int omode,
 		inode_fid = v9fs_writeback_fid(dentry);
 		if (IS_ERR(inode_fid)) {
 			err = PTR_ERR(inode_fid);
+			mutex_unlock(&v9inode->v_mutex);
 			goto error;
 		}
 		v9inode->writeback_fid = (void *) inode_fid;
 	}
+	mutex_unlock(&v9inode->v_mutex);
 	/* Since we are opening a file, assign the open fid to the file */
 	filp = lookup_instantiate_filp(nd, dentry, generic_file_open);
 	if (IS_ERR(filp)) {