btrfs: convert data_seqcount to seqcount_mutex_t

By doing so we can associate the sequence counter to the chunk_mutex
for lockdep purposes (compiled-out otherwise), the mutex is otherwise
used on the write side.
Also avoid explicitly disabling preemption around the write region as it
will now be done automatically by the seqcount machinery based on the
lock type.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: Davidlohr Bueso

fs/btrfs/volumes.c

@@ -431,7 +431,7 @@ static struct btrfs_device *__alloc_device(struct btrfs_fs_info *fs_info)
 
 	atomic_set(&dev->reada_in_flight, 0);
 	atomic_set(&dev->dev_stats_ccnt, 0);
-	btrfs_device_data_ordered_init(dev);
+	btrfs_device_data_ordered_init(dev, fs_info);
 	INIT_RADIX_TREE(&dev->reada_zones, GFP_NOFS & ~__GFP_DIRECT_RECLAIM);
 	INIT_RADIX_TREE(&dev->reada_extents, GFP_NOFS & ~__GFP_DIRECT_RECLAIM);
 	extent_io_tree_init(fs_info, &dev->alloc_state,

fs/btrfs/volumes.h

@@ -39,10 +39,10 @@ struct btrfs_io_geometry {
 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 #include <linux/seqlock.h>
 #define __BTRFS_NEED_DEVICE_DATA_ORDERED
-#define btrfs_device_data_ordered_init(device)	\
-	seqcount_init(&device->data_seqcount)
+#define btrfs_device_data_ordered_init(device, info)				\
+	seqcount_mutex_init(&device->data_seqcount, &info->chunk_mutex)
 #else
-#define btrfs_device_data_ordered_init(device) do { } while (0)
+#define btrfs_device_data_ordered_init(device, info) do { } while (0)
 #endif
 
 #define BTRFS_DEV_STATE_WRITEABLE	(0)
@@ -72,7 +72,8 @@ struct btrfs_device {
 	blk_status_t last_flush_error;
 
 #ifdef __BTRFS_NEED_DEVICE_DATA_ORDERED
-	seqcount_t data_seqcount;
+	/* A seqcount_t with associated chunk_mutex (for lockdep) */
+	seqcount_mutex_t data_seqcount;
 #endif
 
 	/* the internal btrfs device id */
@@ -163,11 +164,9 @@ btrfs_device_get_##name(const struct btrfs_device *dev)			\
 static inline void							\
 btrfs_device_set_##name(struct btrfs_device *dev, u64 size)		\
 {									\
-	preempt_disable();						\
 	write_seqcount_begin(&dev->data_seqcount);			\
 	dev->name = size;						\
 	write_seqcount_end(&dev->data_seqcount);			\
-	preempt_enable();						\
 }
 #elif BITS_PER_LONG==32 && defined(CONFIG_PREEMPTION)
 #define BTRFS_DEVICE_GETSET_FUNCS(name)					\