[dm-devel] [PATCH] dm: fix race condition on Alpha

[Mikulas Patocka]

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 at redhat.com>
Fixes: bd4a6dd241ae ("dm: reduce size of dm_io and dm_target_io structs")

---
 drivers/md/dm-core.h      |    4 ++--
 include/linux/blk_types.h |    2 ++
 2 files changed, 4 insertions(+), 2 deletions(-)

Index: linux-2.6/include/linux/blk_types.h
===================================================================
--- linux-2.6.orig/include/linux/blk_types.h	2022-03-28 18:11:15.000000000 +0200
+++ linux-2.6/include/linux/blk_types.h	2022-03-28 18:20:23.000000000 +0200
@@ -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)
Index: linux-2.6/drivers/md/dm-core.h
===================================================================
--- linux-2.6.orig/drivers/md/dm-core.h	2022-03-28 18:11:11.000000000 +0200
+++ linux-2.6/drivers/md/dm-core.h	2022-03-28 18:21:34.000000000 +0200
@@ -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(struc
 #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;