[PATCH 06/13] loop: Protect all lo_backing_file accesses with lo->lo_mutex

[Bart Van Assche <bvanassche@acm.org>]

Protect all lo_backing_file accesses from the control path with
lo->lo_mutex. Use READ_ONCE() to read lo_backing_file from the data path.
Serialization of I/O path lo_backing_file reads and control path
lo_backing_file changes happens by freezing the request queue.

Remove lo_lock because it is no longer used.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/block/loop.c | 45 ++++++++++++++++++++++++++------------------
 1 file changed, 27 insertions(+), 18 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index aa40a6ed7f35..c5f026520836 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -53,14 +53,13 @@ struct loop_device {
 	int		lo_flags;
 	char		lo_file_name[LO_NAME_SIZE];
 
-	struct file	*lo_backing_file;
+	struct file	*lo_backing_file __guarded_by(&lo_mutex);
 	unsigned int	lo_min_dio_size;
 	unsigned int	lo_dio_mem_align;
 	struct block_device *lo_device;
 
 	gfp_t		old_gfp_mask;
 
-	spinlock_t		lo_lock;
 	int			lo_state;
 	spinlock_t              lo_work_lock;
 	struct workqueue_struct *workqueue;
@@ -261,7 +260,7 @@ static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
 	 * We use fallocate to manipulate the space mappings used by the image
 	 * a.k.a. discard/zerorange.
 	 */
-	struct file *file = lo->lo_backing_file;
+	struct file *file = context_unsafe(READ_ONCE(lo->lo_backing_file));
 	int ret;
 
 	mode |= FALLOC_FL_KEEP_SIZE;
@@ -285,7 +284,7 @@ static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
 
 static int lo_req_flush(struct loop_device *lo, struct request *rq)
 {
-	int ret = vfs_fsync(lo->lo_backing_file, 0);
+	int ret = vfs_fsync(context_unsafe(READ_ONCE(lo->lo_backing_file)), 0);
 	if (unlikely(ret && ret != -EINVAL))
 		ret = -EIO;
 
@@ -354,7 +353,7 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 	struct iov_iter iter;
 	struct req_iterator rq_iter;
 	struct request *rq = blk_mq_rq_from_pdu(cmd);
-	struct file *file = lo->lo_backing_file;
+	struct file *file = context_unsafe(READ_ONCE(lo->lo_backing_file));
 	unsigned int nr_bvec;
 	int ret;
 
@@ -511,6 +510,19 @@ static inline bool is_loop_device(struct file *file)
 	return bdev && bdev->bd_disk->major == LOOP_MAJOR;
 }
 
+static struct file *loop_get_backing_file(struct loop_device *lo)
+	__must_hold(&lo->lo_mutex)
+{
+	if (lo->lo_state != Lo_bound)
+		return NULL;
+	/*
+	 * Order wrt setting lo->lo_backing_file in
+	 * loop_configure().
+	 */
+	rmb();
+	return lo->lo_backing_file;
+}
+
 /* Returns 0 if and only if @file is not backed by loop device @bdev. */
 static int loop_validate_file(struct loop_device *lo, struct file *file,
 			      struct block_device *bdev)
@@ -532,11 +544,10 @@ static int loop_validate_file(struct loop_device *lo, struct file *file,
 			return -EBADF;
 
 		l = f_bdev->bd_disk->private_data;
-		if (l->lo_state != Lo_bound)
+		scoped_guard(mutex, &l->lo_mutex)
+			f = loop_get_backing_file(l);
+		if (!f)
 			return -EINVAL;
-		/* Order wrt setting lo->lo_backing_file in loop_configure(). */
-		rmb();
-		f = l->lo_backing_file;
 	}
 	return 0;
 }
@@ -693,10 +704,9 @@ static ssize_t loop_attr_backing_file_show(struct loop_device *lo, char *buf)
 	ssize_t ret;
 	char *p = NULL;
 
-	spin_lock_irq(&lo->lo_lock);
-	if (lo->lo_backing_file)
-		p = file_path(lo->lo_backing_file, buf, PAGE_SIZE - 1);
-	spin_unlock_irq(&lo->lo_lock);
+	scoped_guard(mutex, &lo->lo_mutex)
+		if (lo->lo_backing_file)
+			p = file_path(lo->lo_backing_file, buf, PAGE_SIZE - 1);
 
 	if (IS_ERR_OR_NULL(p))
 		ret = PTR_ERR(p);
@@ -1174,10 +1184,10 @@ static void __loop_clr_fd(struct loop_device *lo)
 	gfp_t gfp = lo->old_gfp_mask;
 	int err;
 
-	spin_lock_irq(&lo->lo_lock);
-	filp = lo->lo_backing_file;
-	lo->lo_backing_file = NULL;
-	spin_unlock_irq(&lo->lo_lock);
+	scoped_guard(mutex, &lo->lo_mutex) {
+		filp = lo->lo_backing_file;
+		lo->lo_backing_file = NULL;
+	}
 
 	lo->lo_device = NULL;
 	lo->lo_offset = 0;
@@ -2134,7 +2144,6 @@ static int loop_add(int i)
 	lockdep_register_key(&lo->lo_mutex_key);
 	mutex_init_with_key(&lo->lo_mutex, &lo->lo_mutex_key);
 	lo->lo_number		= i;
-	spin_lock_init(&lo->lo_lock);
 	spin_lock_init(&lo->lo_work_lock);
 	INIT_WORK(&lo->rootcg_work, loop_rootcg_workfn);
 	INIT_LIST_HEAD(&lo->rootcg_cmd_list);