drbd: Improve how conflicting writes are handled

The previous algorithm for dealing with overlapping concurrent writes
was generating unnecessary warnings for scenarios which could be
legitimate, and did not always handle partially overlapping requests
correctly.  Improve it algorithm as follows:

* While local or remote write requests are in progress, conflicting new
  local write requests will be delayed (commit 82172f7).

* When a conflict between a local and remote write request is detected,
  the node with the discard flag decides how to resolve the conflict: It
  will ask its peer to discard conflicting requests which are fully
  contained in the local request and retry requests which overlap only
  partially.  This involves a protocol change.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/drivers/block/drbd/drbd_req.h b/drivers/block/drbd/drbd_req.h
index e6f2361d6b19..0b3cd412d52d 100644
--- a/drivers/block/drbd/drbd_req.h
+++ b/drivers/block/drbd/drbd_req.h
@@ -97,7 +97,8 @@ enum drbd_req_event {
         RECV_ACKED_BY_PEER,
         WRITE_ACKED_BY_PEER,
         WRITE_ACKED_BY_PEER_AND_SIS, /* and set_in_sync */
-        CONFLICT_DISCARDED_BY_PEER,
+        DISCARD_WRITE,
+        POSTPONE_WRITE,
         NEG_ACKED,
         BARRIER_ACKED, /* in protocol A and B */
         DATA_RECEIVED, /* (remote read) */
@@ -194,6 +195,9 @@ enum drbd_req_state_bits {
 
         /* Should call drbd_al_complete_io() for this request... */
         __RQ_IN_ACT_LOG,
+
+        /* The peer has sent a retry ACK */
+        __RQ_POSTPONED,
 };
 
 #define RQ_LOCAL_PENDING   (1UL << __RQ_LOCAL_PENDING)
@@ -214,6 +218,7 @@ enum drbd_req_state_bits {
 
 #define RQ_WRITE           (1UL << __RQ_WRITE)
 #define RQ_IN_ACT_LOG      (1UL << __RQ_IN_ACT_LOG)
+#define RQ_POSTPONED       (1UL << __RQ_POSTPONED)
 
 /* For waking up the frozen transfer log mod_req() has to return if the request
    should be counted in the epoch object*/