consolidate the reassignments of ->f_op in ->open() instances

drivers/char/misc.c

@@ -114,7 +114,7 @@ static int misc_open(struct inode * inode, struct file * file)
 	int minor = iminor(inode);
 	struct miscdevice *c;
 	int err = -ENODEV;
-	const struct file_operations *old_fops, *new_fops = NULL;
+	const struct file_operations *new_fops = NULL;
 
 	mutex_lock(&misc_mtx);
 	
@@ -141,17 +141,11 @@ static int misc_open(struct inode * inode, struct file * file)
 	}
 
 	err = 0;
-	old_fops = file->f_op;
-	file->f_op = new_fops;
+	replace_fops(file, new_fops);
 	if (file->f_op->open) {
 		file->private_data = c;
-		err=file->f_op->open(inode,file);
-		if (err) {
-			fops_put(file->f_op);
-			file->f_op = fops_get(old_fops);
-		}
+		err = file->f_op->open(inode,file);
 	}
-	fops_put(old_fops);
 fail:
 	mutex_unlock(&misc_mtx);
 	return err;

drivers/gpu/drm/drm_fops.c

@@ -148,7 +148,7 @@ int drm_stub_open(struct inode *inode, struct file *filp)
 	struct drm_minor *minor;
 	int minor_id = iminor(inode);
 	int err = -ENODEV;
-	const struct file_operations *old_fops;
+	const struct file_operations *new_fops;
 
 	DRM_DEBUG("\n");
 
@@ -163,18 +163,13 @@ int drm_stub_open(struct inode *inode, struct file *filp)
 	if (drm_device_is_unplugged(dev))
 		goto out;
 
-	old_fops = filp->f_op;
-	filp->f_op = fops_get(dev->driver->fops);
-	if (filp->f_op == NULL) {
-		filp->f_op = old_fops;
+	new_fops = fops_get(dev->driver->fops);
+	if (!new_fops)
 		goto out;
-	}
-	if (filp->f_op->open && (err = filp->f_op->open(inode, filp))) {
-		fops_put(filp->f_op);
-		filp->f_op = fops_get(old_fops);
-	}
-	fops_put(old_fops);
 
+	replace_fops(filp, new_fops);
+	if (filp->f_op->open)
+		err = filp->f_op->open(inode, filp);
 out:
 	mutex_unlock(&drm_global_mutex);
 	return err;

drivers/media/dvb-core/dvbdev.c

@@ -74,22 +74,15 @@ static int dvb_device_open(struct inode *inode, struct file *file)
 
 	if (dvbdev && dvbdev->fops) {
 		int err = 0;
-		const struct file_operations *old_fops;
+		const struct file_operations *new_fops;
 
-		file->private_data = dvbdev;
-		old_fops = file->f_op;
-		file->f_op = fops_get(dvbdev->fops);
-		if (file->f_op == NULL) {
-			file->f_op = old_fops;
+		new_fops = fops_get(dvbdev->fops);
+		if (!new_fops)
 			goto fail;
-		}
-		if(file->f_op->open)
+		file->private_data = dvbdev;
+		replace_fops(file, new_fops);
+		if (file->f_op->open)
 			err = file->f_op->open(inode,file);
-		if (err) {
-			fops_put(file->f_op);
-			file->f_op = fops_get(old_fops);
-		}
-		fops_put(old_fops);
 		up_read(&minor_rwsem);
 		mutex_unlock(&dvbdev_mutex);
 		return err;

drivers/usb/core/file.c

@@ -29,27 +29,19 @@ static DECLARE_RWSEM(minor_rwsem);
 
 static int usb_open(struct inode * inode, struct file * file)
 {
-	int minor = iminor(inode);
-	const struct file_operations *c;
 	int err = -ENODEV;
-	const struct file_operations *old_fops, *new_fops = NULL;
+	const struct file_operations *new_fops;
 
 	down_read(&minor_rwsem);
-	c = usb_minors[minor];
+	new_fops = fops_get(usb_minors[iminor(inode)]);
 
-	if (!c || !(new_fops = fops_get(c)))
+	if (!new_fops)
 		goto done;
 
-	old_fops = file->f_op;
-	file->f_op = new_fops;
+	replace_fops(file, new_fops);
 	/* Curiouser and curiouser... NULL ->open() as "no device" ? */
 	if (file->f_op->open)
 		err = file->f_op->open(inode,file);
-	if (err) {
-		fops_put(file->f_op);
-		file->f_op = fops_get(old_fops);
-	}
-	fops_put(old_fops);
  done:
 	up_read(&minor_rwsem);
 	return err;

fs/char_dev.c

@@ -368,6 +368,7 @@ void cdev_put(struct cdev *p)
  */
 static int chrdev_open(struct inode *inode, struct file *filp)
 {
+	const struct file_operations *fops;
 	struct cdev *p;
 	struct cdev *new = NULL;
 	int ret = 0;
@@ -400,10 +401,11 @@ static int chrdev_open(struct inode *inode, struct file *filp)
 		return ret;
 
 	ret = -ENXIO;
-	filp->f_op = fops_get(p->ops);
-	if (!filp->f_op)
+	fops = fops_get(p->ops);
+	if (!fops)
 		goto out_cdev_put;
 
+	replace_fops(filp, fops);
 	if (filp->f_op->open) {
 		ret = filp->f_op->open(inode, filp);
 		if (ret)

include/linux/fs.h

@@ -1875,6 +1875,17 @@ extern struct dentry *mount_pseudo(struct file_system_type *, char *,
 	(((fops) && try_module_get((fops)->owner) ? (fops) : NULL))
 #define fops_put(fops) \
 	do { if (fops) module_put((fops)->owner); } while(0)
+/*
+ * This one is to be used *ONLY* from ->open() instances.
+ * fops must be non-NULL, pinned down *and* module dependencies
+ * should be sufficient to pin the caller down as well.
+ */
+#define replace_fops(f, fops) \
+	do {	\
+		struct file *__file = (f); \
+		fops_put(__file->f_op); \
+		BUG_ON(!(__file->f_op = (fops))); \
+	} while(0)
 
 extern int register_filesystem(struct file_system_type *);
 extern int unregister_filesystem(struct file_system_type *);

sound/core/sound.c

@@ -153,7 +153,7 @@ static int snd_open(struct inode *inode, struct file *file)
 {
 	unsigned int minor = iminor(inode);
 	struct snd_minor *mptr = NULL;
-	const struct file_operations *old_fops;
+	const struct file_operations *new_fops;
 	int err = 0;
 
 	if (minor >= ARRAY_SIZE(snd_minors))
@@ -167,24 +167,14 @@ static int snd_open(struct inode *inode, struct file *file)
 			return -ENODEV;
 		}
 	}
-	old_fops = file->f_op;
-	file->f_op = fops_get(mptr->f_ops);
-	if (file->f_op == NULL) {
-		file->f_op = old_fops;
-		err = -ENODEV;
-	}
+	new_fops = fops_get(mptr->f_ops);
 	mutex_unlock(&sound_mutex);
-	if (err < 0)
-		return err;
+	if (!new_fops)
+		return -ENODEV;
+	replace_fops(file, new_fops);
 
-	if (file->f_op->open) {
+	if (file->f_op->open)
 		err = file->f_op->open(inode, file);
-		if (err) {
-			fops_put(file->f_op);
-			file->f_op = fops_get(old_fops);
-		}
-	}
-	fops_put(old_fops);
 	return err;
 }
 

sound/sound_core.c

@@ -626,31 +626,20 @@ static int soundcore_open(struct inode *inode, struct file *file)
 		if (s)
 			new_fops = fops_get(s->unit_fops);
 	}
+	spin_unlock(&sound_loader_lock);
 	if (new_fops) {
 		/*
 		 * We rely upon the fact that we can't be unloaded while the
-		 * subdriver is there, so if ->open() is successful we can
-		 * safely drop the reference counter and if it is not we can
-		 * revert to old ->f_op. Ugly, indeed, but that's the cost of
-		 * switching ->f_op in the first place.
+		 * subdriver is there.
 		 */
 		int err = 0;
-		const struct file_operations *old_fops = file->f_op;
-		file->f_op = new_fops;
-		spin_unlock(&sound_loader_lock);
+		replace_fops(file, new_fops);
 
 		if (file->f_op->open)
 			err = file->f_op->open(inode,file);
 
-		if (err) {
-			fops_put(file->f_op);
-			file->f_op = fops_get(old_fops);
-		}
-
-		fops_put(old_fops);
 		return err;
 	}
-	spin_unlock(&sound_loader_lock);
 	return -ENODEV;
 }