dm: fix dm_io and dm_target_io flags race condition on Alpha

Early alpha processors cannot write a single byte or short; they read 8
bytes, modify the value in registers and write back 8 bytes.

This could cause race condition in the structure dm_io - if the fields
flags and io_count are modified simultaneously.

Fix this bug by using 32-bit flags if we are on Alpha and if we are
compiling for a processor that doesn't have the byte-word-extension.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: bd4a6dd241 ("dm: reduce size of dm_io and dm_target_io structs")
[snitzer: Jens allowed this change since Mikulas owns a relevant Alpha!]
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-core.h

@@ -210,7 +210,7 @@ struct dm_table {
 #define DM_TIO_MAGIC 28714
 struct dm_target_io {
 	unsigned short magic;
-	unsigned short flags;
+	blk_short_t flags;
 	unsigned int target_bio_nr;
 	struct dm_io *io;
 	struct dm_target *ti;
@@ -244,7 +244,7 @@ static inline void dm_tio_set_flag(struct dm_target_io *tio, unsigned int bit)
 #define DM_IO_MAGIC 19577
 struct dm_io {
 	unsigned short magic;
-	unsigned short flags;
+	blk_short_t flags;
 	atomic_t io_count;
 	struct mapped_device *md;
 	struct bio *orig_bio;

include/linux/blk_types.h

@@ -85,8 +85,10 @@ struct block_device {
  */
 #if defined(CONFIG_ALPHA) && !defined(__alpha_bwx__)
 typedef u32 __bitwise blk_status_t;
+typedef u32 blk_short_t;
 #else
 typedef u8 __bitwise blk_status_t;
+typedef u16 blk_short_t;
 #endif
 #define	BLK_STS_OK 0
 #define BLK_STS_NOTSUPP		((__force blk_status_t)1)