dm thin metadata: don't issue prefetches if a transaction abort has failed

If a transaction abort has failed then we can no longer use the metadata
device. Typically this happens if the superblock is unreadable.

This fix addresses a crash seen during metadata device failure testing.

Fixes: 8a01a6af75 ("dm thin: prefetch missing metadata pages")
Cc: [e-mail] # 3.19+
Signed-off-by: Joe Thornber <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-thin-metadata.c

···
 
 void dm_pool_issue_prefetches(struct dm_pool_metadata *pmd)
 {
-	dm_tm_issue_prefetches(pmd->tm);
+	down_read(&pmd->root_lock);
+	if (!pmd->fail_io)
+		dm_tm_issue_prefetches(pmd->tm);
+	up_read(&pmd->root_lock);
 }