[PATCH] block: use 32-bit blk_status_t on Alpha

[Mikulas Patocka]

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

The type blk_status_t is defined as one byte, it is often written
asynchronously by I/O completion routines, this asynchronous modification
can corrupt content of nearby bytes if these nearby bytes can be written
simultaneously by another CPU.

- one example of such corruption is the structure dm_io where
  "blk_status_t status" is written by an asynchronous completion routine
  and "atomic_t io_count" is modified synchronously
- another example is the structure dm_buffer where "unsigned hold_count"
  is modified synchronously from process context and "blk_status_t
  write_error" is modified asynchronously from bio completion routine

This patch fixes the bug by changing the type blk_status_t to 32 bits 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@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx	# 4.13+

---
 include/linux/blk_types.h |    5 +++++
 1 file changed, 5 insertions(+)

Index: linux-2.6/include/linux/blk_types.h
===================================================================
--- linux-2.6.orig/include/linux/blk_types.h	2018-02-14 20:24:42.038255000 +0100
+++ linux-2.6/include/linux/blk_types.h	2018-03-21 15:04:54.969999000 +0100
@@ -20,8 +20,13 @@ typedef void (bio_end_io_t) (struct bio
 
 /*
  * Block error status values.  See block/blk-core:blk_errors for the details.
+ * Alpha cannot write a byte atomically, so we need to use 32-bit value.
  */
+#if defined(CONFIG_ALPHA) && !defined(__alpha_bwx__)
+typedef u32 __bitwise blk_status_t;
+#else
 typedef u8 __bitwise blk_status_t;
+#endif
 #define	BLK_STS_OK 0
 #define BLK_STS_NOTSUPP		((__force blk_status_t)1)
 #define BLK_STS_TIMEOUT		((__force blk_status_t)2)