fuse: fix parallel dio write on file open in passthrough mode

Parallel dio write takes a negative refcount of fi->iocachectr and so does
open of file in passthrough mode.

The refcount of passthrough mode is associated with attach/detach of a
fuse_backing object to fuse inode.

For parallel dio write, the backing file is irrelevant, so the call to
fuse_inode_uncached_io_start() passes a NULL fuse_backing object.

Passing a NULL fuse_backing will result in false -EBUSY error if the file
is already open in passthrough mode.

Allow taking negative fi->iocachectr refcount with NULL fuse_backing,
because it does not conflict with an already attached fuse_backing object.

Fixes: 4a90451 ("fuse: implement open in passthrough mode")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: amir73il

fs/fuse/iomode.c

@@ -90,7 +90,7 @@ int fuse_inode_uncached_io_start(struct fuse_inode *fi, struct fuse_backing *fb)
 	spin_lock(&fi->lock);
 	/* deny conflicting backing files on same fuse inode */
 	oldfb = fuse_inode_backing(fi);
-	if (oldfb && oldfb != fb) {
+	if (fb && oldfb && oldfb != fb) {
 		err = -EBUSY;
 		goto unlock;
 	}
@@ -101,7 +101,7 @@ int fuse_inode_uncached_io_start(struct fuse_inode *fi, struct fuse_backing *fb)
 	fi->iocachectr--;
 
 	/* fuse inode holds a single refcount of backing file */
-	if (!oldfb) {
+	if (fb && !oldfb) {
 		oldfb = fuse_inode_backing_set(fi, fb);
 		WARN_ON_ONCE(oldfb != NULL);
 	} else {