This will make it possible to grab the fuse_dev and subsequently release
the file that it came from.

In the above case, fud->fc will be set to FUSE_DEV_FC_DISCONNECTED to
indicate that this is no longer a functional device.

When trying to assign an fc to such a disconnected fuse_dev, the fc is set
to the disconnected state.

Use atomic operations xchg() and cmpxchg() to prevent races.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 fs/fuse/cuse.c       |  2 +-
 fs/fuse/dev.c        |  9 +++++++--
 fs/fuse/fuse_dev_i.h | 11 ++++-------
 fs/fuse/fuse_i.h     |  6 ++++--
 fs/fuse/inode.c      | 43 +++++++++++++++++++++++++++++++++++--------
 fs/fuse/virtio_fs.c  |  2 +-
 6 files changed, 52 insertions(+), 21 deletions(-)

diff --git a/fs/fuse/cuse.c b/fs/fuse/cuse.c
index dfcb98a654d8..174333633471 100644
--- a/fs/fuse/cuse.c
+++ b/fs/fuse/cuse.c
@@ -527,7 +527,7 @@ static int cuse_channel_open(struct inode *inode, struct file *file)
 	cc->fc.initialized = 1;
 	rc = cuse_send_init(cc);
 	if (rc) {
-		fuse_dev_free(fud);
+		fuse_dev_put(fud);
 		return rc;
 	}
 	file->private_data = fud;
diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 4795e3d01b75..3adf6bd38c9b 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -2527,7 +2527,8 @@ void fuse_wait_aborted(struct fuse_conn *fc)
 int fuse_dev_release(struct inode *inode, struct file *file)
 {
 	struct fuse_dev *fud = fuse_file_to_fud(file);
-	struct fuse_conn *fc = fuse_dev_fc_get(fud);
+	/* Pairs with cmpxchg() in fuse_dev_install() */
+	struct fuse_conn *fc = xchg(&fud->fc, FUSE_DEV_FC_DISCONNECTED);
 
 	if (fc) {
 		struct fuse_pqueue *fpq = &fud->pq;
@@ -2547,8 +2548,12 @@ int fuse_dev_release(struct inode *inode, struct file *file)
 			WARN_ON(fc->iq.fasync != NULL);
 			fuse_abort_conn(fc);
 		}
+		spin_lock(&fc->lock);
+		list_del(&fud->entry);
+		spin_unlock(&fc->lock);
+		fuse_conn_put(fc);
 	}
-	fuse_dev_free(fud);
+	fuse_dev_put(fud);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(fuse_dev_release);
diff --git a/fs/fuse/fuse_dev_i.h b/fs/fuse/fuse_dev_i.h
index 522b2012cd1f..9eae14ace73c 100644
--- a/fs/fuse/fuse_dev_i.h
+++ b/fs/fuse/fuse_dev_i.h
@@ -39,22 +39,19 @@ struct fuse_copy_state {
 	} ring;
 };
 
+/* fud->fc gets assigned to this value when /dev/fuse is closed */
+#define FUSE_DEV_FC_DISCONNECTED ((struct fuse_conn *) 1)
+
 /*
  * Lockless access is OK, because fud->fc is set once during mount and is valid
  * until the file is released.
  */
 static inline struct fuse_conn *fuse_dev_fc_get(struct fuse_dev *fud)
 {
-	/* Pairs with smp_store_release() in fuse_dev_fc_set() */
+	/* Pairs with xchg() in fuse_dev_install() */
 	return smp_load_acquire(&fud->fc);
 }
 
-static inline void fuse_dev_fc_set(struct fuse_dev *fud, struct fuse_conn *fc)
-{
-	/* Pairs with smp_load_acquire() in fuse_dev_fc_get() */
-	smp_store_release(&fud->fc, fc);
-}
-
 static inline struct fuse_dev *fuse_file_to_fud(struct file *file)
 {
 	return file->private_data;
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 739220d96b6f..66ec92f06497 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -576,6 +576,8 @@ struct fuse_pqueue {
  * Fuse device instance
  */
 struct fuse_dev {
+	refcount_t ref;
+
 	bool sync_init;
 
 	/** Fuse connection for this device */
@@ -1341,8 +1343,8 @@ void fuse_conn_put(struct fuse_conn *fc);
 
 struct fuse_dev *fuse_dev_alloc_install(struct fuse_conn *fc);
 struct fuse_dev *fuse_dev_alloc(void);
-void fuse_dev_install(struct fuse_dev *fud, struct fuse_conn *fc);
-void fuse_dev_free(struct fuse_dev *fud);
+bool fuse_dev_install(struct fuse_dev *fud, struct fuse_conn *fc);
+void fuse_dev_put(struct fuse_dev *fud);
 int fuse_send_init(struct fuse_mount *fm);
 
 /**
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 53663846db36..ba845092e7f2 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1622,6 +1622,7 @@ struct fuse_dev *fuse_dev_alloc(void)
 	if (!fud)
 		return NULL;
 
+	refcount_set(&fud->ref, 1);
 	pq = kzalloc_objs(struct list_head, FUSE_PQ_HASH_SIZE);
 	if (!pq) {
 		kfree(fud);
@@ -1635,12 +1636,32 @@ struct fuse_dev *fuse_dev_alloc(void)
 }
 EXPORT_SYMBOL_GPL(fuse_dev_alloc);
 
-void fuse_dev_install(struct fuse_dev *fud, struct fuse_conn *fc)
+bool fuse_dev_install(struct fuse_dev *fud, struct fuse_conn *fc)
 {
-	fuse_dev_fc_set(fud, fuse_conn_get(fc));
+	struct fuse_conn *old_fc;
+
+	fuse_conn_get(fc);
 	spin_lock(&fc->lock);
+	/*
+	 * Pairs with:
+	 *  - xchg() in fuse_dev_release()
+	 *  - smp_load_acquire() in fuse_dev_fc_get()
+	 */
+	old_fc = cmpxchg(&fud->fc, NULL, fc);
+	if (old_fc) {
+		/*
+		 * failed to set fud->fc because
+		 *  - it was already set to a different fc
+		 *  - it was set to disconneted
+		 */
+		spin_unlock(&fc->lock);
+		fuse_conn_put(fc);
+		return false;
+	}
 	list_add_tail(&fud->entry, &fc->devices);
 	spin_unlock(&fc->lock);
+
+	return true;
 }
 EXPORT_SYMBOL_GPL(fuse_dev_install);
 
@@ -1657,11 +1678,15 @@ struct fuse_dev *fuse_dev_alloc_install(struct fuse_conn *fc)
 }
 EXPORT_SYMBOL_GPL(fuse_dev_alloc_install);
 
-void fuse_dev_free(struct fuse_dev *fud)
+void fuse_dev_put(struct fuse_dev *fud)
 {
-	struct fuse_conn *fc = fuse_dev_fc_get(fud);
+	struct fuse_conn *fc;
 
-	if (fc) {
+	if (!refcount_dec_and_test(&fud->ref))
+		return;
+
+	fc = fuse_dev_fc_get(fud);
+	if (fc && fc != FUSE_DEV_FC_DISCONNECTED) {
 		spin_lock(&fc->lock);
 		list_del(&fud->entry);
 		spin_unlock(&fc->lock);
@@ -1671,7 +1696,7 @@ void fuse_dev_free(struct fuse_dev *fud)
 	kfree(fud->pq.processing);
 	kfree(fud);
 }
-EXPORT_SYMBOL_GPL(fuse_dev_free);
+EXPORT_SYMBOL_GPL(fuse_dev_put);
 
 static void fuse_fill_attr_from_inode(struct fuse_attr *attr,
 				      const struct fuse_inode *fi)
@@ -1900,8 +1925,10 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
 	list_add_tail(&fc->entry, &fuse_conn_list);
 	sb->s_root = root_dentry;
 	if (fud) {
-		fuse_dev_install(fud, fc);
-		wake_up_all(&fuse_dev_waitq);
+		if (!fuse_dev_install(fud, fc))
+			fc->connected = 0; /* device file got closed */
+		else
+			wake_up_all(&fuse_dev_waitq);
 	}
 	mutex_unlock(&fuse_mutex);
 	return 0;
diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index f685916754ad..12300651a0f1 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -486,7 +486,7 @@ static void virtio_fs_free_devs(struct virtio_fs *fs)
 		if (!fsvq->fud)
 			continue;
 
-		fuse_dev_free(fsvq->fud);
+		fuse_dev_put(fsvq->fud);
 		fsvq->fud = NULL;
 	}
 }
-- 
2.53.0