drbd: base completion and destruction of requests on ref counts

cherry-picked and adapted from drbd 9 devel branch

The logic for when to get or put a reference is in mod_rq_state().

To not get confused in the freeze/thaw respectively resend/restart
paths, or when cleaning up requests waiting for P_BARRIER_ACK, this
also introduces additional state flags:
RQ_COMPLETION_SUSP, and RQ_EXP_BARR_ACK.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_int.h

@@ -575,13 +575,14 @@ struct drbd_request {
 
 	struct list_head tl_requests; /* ring list in the transfer log */
 	struct bio *master_bio;       /* master bio pointer */
-	unsigned long rq_state; /* see comments above _req_mod() */
 	unsigned long start_time;
 
 	/* once it hits 0, we may complete the master_bio */
 	atomic_t completion_ref;
 	/* once it hits 0, we may destroy this drbd_request object */
 	struct kref kref;
+
+	unsigned rq_state; /* see comments above _req_mod() */
 };
 
 struct drbd_epoch {

drivers/block/drbd/drbd_main.c

@@ -210,7 +210,7 @@ void tl_release(struct drbd_tconn *tconn, unsigned int barrier_nr,
 	/* find latest not yet barrier-acked write request,
 	 * count writes in its epoch. */
 	list_for_each_entry(r, &tconn->transfer_log, tl_requests) {
-		const unsigned long s = r->rq_state;
+		const unsigned s = r->rq_state;
 		if (!req) {
 			if (!(s & RQ_WRITE))
 				continue;

drivers/block/drbd/drbd_req.h

@@ -203,11 +203,18 @@ enum drbd_req_state_bits {
 	/* The peer has sent a retry ACK */
 	__RQ_POSTPONED,
 
+	/* would have been completed,
+	 * but was not, because of drbd_suspended() */
+	__RQ_COMPLETION_SUSP,
+
 	/* We expect a receive ACK (wire proto B) */
 	__RQ_EXP_RECEIVE_ACK,
 
 	/* We expect a write ACK (wite proto C) */
 	__RQ_EXP_WRITE_ACK,
+
+	/* waiting for a barrier ack, did an extra kref_get */
+	__RQ_EXP_BARR_ACK,
 };
 
 #define RQ_LOCAL_PENDING   (1UL << __RQ_LOCAL_PENDING)
@@ -230,8 +237,10 @@ 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)
+#define RQ_COMPLETION_SUSP (1UL << __RQ_COMPLETION_SUSP)
 #define RQ_EXP_RECEIVE_ACK (1UL << __RQ_EXP_RECEIVE_ACK)
 #define RQ_EXP_WRITE_ACK   (1UL << __RQ_EXP_WRITE_ACK)
+#define RQ_EXP_BARR_ACK    (1UL << __RQ_EXP_BARR_ACK)
 
 /* For waking up the frozen transfer log mod_req() has to return if the request
    should be counted in the epoch object*/