[PATCH 3.18-4.7] dm io: fix duplicate bio completion due to missing ref count

[Mikulas Patocka <mpatocka@redhat.com>]

Hi

This is backport of the upstream commit that fixes memory corruption in 
dm-io. It is suitable for stable kernels 3.18 to 4.7. (the bug didn't 
exist before 3.18)

Mikulas


commit feb7695fe9fb83084aa29de0094774f4c9d4c9fc
Author: Mike Snitzer <snitzer@redhat.com>
Date:   Tue Jun 20 19:14:30 2017 -0400

    dm io: fix duplicate bio completion due to missing ref count
    
    If only a subset of the devices associated with multiple regions support
    a given special operation (eg. DISCARD) then the dec_count() that is
    used to set error for the region must increment the io->count.
    
    Otherwise, when the dec_count() is called it can cause the dm-io
    caller's bio to be completed multiple times.  As was reported against
    the dm-mirror target that had mirror legs with a mix of discard
    capabilities.
    
    Bug: https://bugzilla.kernel.org/show_bug.cgi?id=196077
    Reported-by: Zhang Yi <yizhan@redhat.com>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>

---
 drivers/md/dm-io.c |    1 +
 1 file changed, 1 insertion(+)

Index: linux-stable/drivers/md/dm-io.c
===================================================================
--- linux-stable.orig/drivers/md/dm-io.c	2018-03-06 14:13:59.000000000 +0100
+++ linux-stable/drivers/md/dm-io.c	2018-03-06 14:14:23.000000000 +0100
@@ -316,6 +316,7 @@ static void do_region(int op, int op_fla
 		special_cmd_max_sectors = q->limits.max_write_same_sectors;
 	if ((op == REQ_OP_DISCARD || op == REQ_OP_WRITE_SAME) &&
 	    special_cmd_max_sectors == 0) {
+		atomic_inc(&io->count);
 		dec_count(io, region, -EOPNOTSUPP);
 		return;
 	}