[PATCH] md/raid5: fix race between reshape and chunk-aligned read

[FengWei Shih <dannyshih@synology.com>]

raid5_make_request() checks mddev->reshape_position to decide whether
to allow chunk-aligned reads. However in raid5_start_reshape(), the
layout configuration (raid_disks, algorithm, etc.) is updated before
mddev->reshape_position is set:

  reshape (raid5_start_reshape)        read (raid5_make_request)
  ==============================       ===========================
  write_seqcount_begin
  update raid_disks, algorithm...
  set conf->reshape_progress
  write_seqcount_end
                                        check mddev->reshape_position
                                          * still MaxSector, allow
                                        raid5_read_one_chunk()
                                          * use new layout
  raid5_quiesce()
  set mddev->reshape_position

Since reshape_position is not yet updated, raid5_make_request()
considers no reshape is in progress and proceeds with the
chunk-aligned path, but the layout has already changed, causing
raid5_compute_sector() to return an incorrect physical address.

Fix this by reading conf->reshape_progress under gen_lock in
raid5_read_one_chunk() and falling back to the stripe path if a
reshape is in progress.

Signed-off-by: FengWei Shih <dannyshih@synology.com>
---
 drivers/md/raid5.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index a8e8d431071b..bded2b86f0ef 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -5421,6 +5421,11 @@ static int raid5_read_one_chunk(struct mddev *mddev, struct bio *raid_bio)
 	sector_t sector, end_sector;
 	int dd_idx;
 	bool did_inc;
+	int seq;
+
+	seq = read_seqcount_begin(&conf->gen_lock);
+	if (unlikely(conf->reshape_progress != MaxSector))
+		return 0;
 
 	if (!in_chunk_boundary(mddev, raid_bio)) {
 		pr_debug("%s: non aligned\n", __func__);
@@ -5431,6 +5436,9 @@ static int raid5_read_one_chunk(struct mddev *mddev, struct bio *raid_bio)
 				      &dd_idx, NULL);
 	end_sector = sector + bio_sectors(raid_bio);
 
+	if (read_seqcount_retry(&conf->gen_lock, seq))
+		return 0;
+
 	if (r5c_big_stripe_cached(conf, sector))
 		return 0;
 
-- 
2.25.1


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