md/raid5: use seqcount to protect access to shape in make_request.

make_request() access various shape parameters (raid_disks, chunk_size
etc) which might be changed by raid5_start_reshape().

If the later is called at and awkward time during the form, the wrong
stripe_head might be used.

So introduce a 'seqcount' and after finding a stripe_head make sure
there is no reason to expect that we got the wrong one.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -4408,8 +4408,10 @@ static void make_request(struct mddev *mddev, struct bio * bi)
 	for (;logical_sector < last_sector; logical_sector += STRIPE_SECTORS) {
 		DEFINE_WAIT(w);
 		int previous;
+		int seq;
 
 	retry:
+		seq = read_seqcount_begin(&conf->gen_lock);
 		previous = 0;
 		prepare_to_wait(&conf->wait_for_overlap, &w, TASK_UNINTERRUPTIBLE);
 		if (unlikely(conf->reshape_progress != MaxSector)) {
@@ -4442,7 +4444,7 @@ static void make_request(struct mddev *mddev, struct bio * bi)
 						  previous,
 						  &dd_idx, NULL);
 		pr_debug("raid456: make_request, sector %llu logical %llu\n",
-			(unsigned long long)new_sector, 
+			(unsigned long long)new_sector,
 			(unsigned long long)logical_sector);
 
 		sh = get_active_stripe(conf, new_sector, previous,
@@ -4471,6 +4473,13 @@ static void make_request(struct mddev *mddev, struct bio * bi)
 					goto retry;
 				}
 			}
+			if (read_seqcount_retry(&conf->gen_lock, seq)) {
+				/* Might have got the wrong stripe_head
+				 * by accident
+				 */
+				release_stripe(sh);
+				goto retry;
+			}
 
 			if (rw == WRITE &&
 			    logical_sector >= mddev->suspend_lo &&
@@ -5437,6 +5446,7 @@ static struct r5conf *setup_conf(struct mddev *mddev)
 	if (alloc_thread_groups(conf, 0))
 		goto abort;
 	spin_lock_init(&conf->device_lock);
+	seqcount_init(&conf->gen_lock);
 	init_waitqueue_head(&conf->wait_for_stripe);
 	init_waitqueue_head(&conf->wait_for_overlap);
 	INIT_LIST_HEAD(&conf->handle_list);
@@ -6249,6 +6259,7 @@ static int raid5_start_reshape(struct mddev *mddev)
 
 	atomic_set(&conf->reshape_stripes, 0);
 	spin_lock_irq(&conf->device_lock);
+	write_seqcount_begin(&conf->gen_lock);
 	conf->previous_raid_disks = conf->raid_disks;
 	conf->raid_disks += mddev->delta_disks;
 	conf->prev_chunk_sectors = conf->chunk_sectors;
@@ -6265,6 +6276,7 @@ static int raid5_start_reshape(struct mddev *mddev)
 	else
 		conf->reshape_progress = 0;
 	conf->reshape_safe = conf->reshape_progress;
+	write_seqcount_end(&conf->gen_lock);
 	spin_unlock_irq(&conf->device_lock);
 
 	/* Add some new drives, as many as will fit.

drivers/md/raid5.h

@@ -400,6 +400,7 @@ struct r5conf {
 	int			prev_chunk_sectors;
 	int			prev_algo;
 	short			generation; /* increments with every reshape */
+	seqcount_t		gen_lock;	/* lock against generation changes */
 	unsigned long		reshape_checkpoint; /* Time we last updated
 						     * metadata */
 	long long		min_offset_diff; /* minimum difference between