[PATCH] md: improvements to raid5 handling of read errors

Two refinements to the 'attempt-overwrite-on-read-error' mechanism.
1/ If the array is read-only, don't attempt an over-write.
2/ If there are more than max_nr_stripes read errors on a device with
   no success, fail the drive.  This will make sure a dead
   drive will be eventually kicked even when we aren't trying
   to rewrite (which would normally kick a dead drive more quickly.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/raid/md_k.h

@@ -134,6 +134,9 @@ struct mdk_rdev_s
 					 * only maintained for arrays that
 					 * support hot removal
 					 */
+	atomic_t	read_errors;	/* number of consecutive read errors that
+					 * we have tried to ignore.
+					 */
 };
 
 typedef struct mdk_personality_s mdk_personality_t;