[PATCH v4 1/2] loop: use READ_ONCE() to read lo->lo_state without locking

[Yongpeng Yang <yangyongpeng.storage@gmail.com>]

From: Yongpeng Yang <yangyongpeng@xiaomi.com>

When lo->lo_mutex is not held, direct access may read stale data. This
patch uses READ_ONCE() to read lo->lo_state and data_race() to silence
code checkers, and changes all assignments to use WRITE_ONCE().

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
---
v4:
- Avoid the overly long line in loop_control_get_free.
v3:
- Use WRITE_ONCE() instead of assignments to update lo_state.
v2:
- Use READ_ONCE() instead of converting lo_state to atomic_t type.
---
 drivers/block/loop.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 272bc608e528..32a3a5b13802 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1082,7 +1082,7 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode,
 	/* Order wrt reading lo_state in loop_validate_file(). */
 	wmb();
 
-	lo->lo_state = Lo_bound;
+	WRITE_ONCE(lo->lo_state, Lo_bound);
 	if (part_shift)
 		lo->lo_flags |= LO_FLAGS_PARTSCAN;
 	partscan = lo->lo_flags & LO_FLAGS_PARTSCAN;
@@ -1179,7 +1179,7 @@ static void __loop_clr_fd(struct loop_device *lo)
 	if (!part_shift)
 		set_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
 	mutex_lock(&lo->lo_mutex);
-	lo->lo_state = Lo_unbound;
+	WRITE_ONCE(lo->lo_state, Lo_unbound);
 	mutex_unlock(&lo->lo_mutex);
 
 	/*
@@ -1218,7 +1218,7 @@ static int loop_clr_fd(struct loop_device *lo)
 
 	lo->lo_flags |= LO_FLAGS_AUTOCLEAR;
 	if (disk_openers(lo->lo_disk) == 1)
-		lo->lo_state = Lo_rundown;
+		WRITE_ONCE(lo->lo_state, Lo_rundown);
 	loop_global_unlock(lo, true);
 
 	return 0;
@@ -1743,7 +1743,7 @@ static void lo_release(struct gendisk *disk)
 
 	mutex_lock(&lo->lo_mutex);
 	if (lo->lo_state == Lo_bound && (lo->lo_flags & LO_FLAGS_AUTOCLEAR))
-		lo->lo_state = Lo_rundown;
+		WRITE_ONCE(lo->lo_state, Lo_rundown);
 
 	need_clear = (lo->lo_state == Lo_rundown);
 	mutex_unlock(&lo->lo_mutex);
@@ -1858,7 +1858,7 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx,
 
 	blk_mq_start_request(rq);
 
-	if (lo->lo_state != Lo_bound)
+	if (data_race(READ_ONCE(lo->lo_state)) != Lo_bound)
 		return BLK_STS_IOERR;
 
 	switch (req_op(rq)) {
@@ -2016,7 +2016,7 @@ static int loop_add(int i)
 	lo->worker_tree = RB_ROOT;
 	INIT_LIST_HEAD(&lo->idle_worker_list);
 	timer_setup(&lo->timer, loop_free_idle_workers_timer, TIMER_DEFERRABLE);
-	lo->lo_state = Lo_unbound;
+	WRITE_ONCE(lo->lo_state, Lo_unbound);
 
 	err = mutex_lock_killable(&loop_ctl_mutex);
 	if (err)
@@ -2174,7 +2174,7 @@ static int loop_control_remove(int idx)
 		goto mark_visible;
 	}
 	/* Mark this loop device as no more bound, but not quite unbound yet */
-	lo->lo_state = Lo_deleting;
+	WRITE_ONCE(lo->lo_state, Lo_deleting);
 	mutex_unlock(&lo->lo_mutex);
 
 	loop_remove(lo);
@@ -2197,8 +2197,12 @@ static int loop_control_get_free(int idx)
 	if (ret)
 		return ret;
 	idr_for_each_entry(&loop_index_idr, lo, id) {
-		/* Hitting a race results in creating a new loop device which is harmless. */
-		if (lo->idr_visible && data_race(lo->lo_state) == Lo_unbound)
+		/*
+		 * Hitting a race results in creating a new loop device
+		 * which is harmless.
+		 */
+		if (lo->idr_visible &&
+		    data_race(READ_ONCE(lo->lo_state)) == Lo_unbound)
 			goto found;
 	}
 	mutex_unlock(&loop_ctl_mutex);
-- 
2.43.0