md/raid5: remove rcu protection to access rdev from conf

Because it's safe to accees rdev from conf:
 - If any spinlock is held, because synchronize_rcu() from
   md_kick_rdev_from_array() will prevent 'rdev' to be freed until
   spinlock is released;
 - If 'reconfig_lock' is held, because rdev can't be added or removed from
   array;
 - If there is normal IO inflight, because mddev_suspend() will prevent
   rdev to be added or removed from array;
 - If there is sync IO inflight, because 'MD_RECOVERY_RUNNING' is
   checked in remove_and_add_spares().

And these will cover all the scenarios in raid456.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231125081604.3939938-5-yukuai1@huaweicloud.com

drivers/md/raid5-cache.c

@@ -1890,28 +1890,22 @@ r5l_recovery_replay_one_stripe(struct r5conf *conf,
 			continue;
 
 		/* in case device is broken */
-		rcu_read_lock();
-		rdev = rcu_dereference(conf->disks[disk_index].rdev);
+		rdev = conf->disks[disk_index].rdev;
 		if (rdev) {
 			atomic_inc(&rdev->nr_pending);
-			rcu_read_unlock();
 			sync_page_io(rdev, sh->sector, PAGE_SIZE,
 				     sh->dev[disk_index].page, REQ_OP_WRITE,
 				     false);
 			rdev_dec_pending(rdev, rdev->mddev);
-			rcu_read_lock();
 		}
-		rrdev = rcu_dereference(conf->disks[disk_index].replacement);
+		rrdev = conf->disks[disk_index].replacement;
 		if (rrdev) {
 			atomic_inc(&rrdev->nr_pending);
-			rcu_read_unlock();
 			sync_page_io(rrdev, sh->sector, PAGE_SIZE,
 				     sh->dev[disk_index].page, REQ_OP_WRITE,
 				     false);
 			rdev_dec_pending(rrdev, rrdev->mddev);
-			rcu_read_lock();
 		}
-		rcu_read_unlock();
 	}
 	ctx->data_parity_stripes++;
 out:
@@ -2948,7 +2942,6 @@ bool r5c_big_stripe_cached(struct r5conf *conf, sector_t sect)
 	if (!log)
 		return false;
 
-	WARN_ON_ONCE(!rcu_read_lock_held());
 	tree_index = r5c_tree_index(conf, sect);
 	slot = radix_tree_lookup(&log->big_stripe_tree, tree_index);
 	return slot != NULL;

drivers/md/raid5-ppl.c

@@ -620,11 +620,9 @@ static void ppl_do_flush(struct ppl_io_unit *io)
 		struct md_rdev *rdev;
 		struct block_device *bdev = NULL;
 
-		rcu_read_lock();
-		rdev = rcu_dereference(conf->disks[i].rdev);
+		rdev = conf->disks[i].rdev;
 		if (rdev && !test_bit(Faulty, &rdev->flags))
 			bdev = rdev->bdev;
-		rcu_read_unlock();
 
 		if (bdev) {
 			struct bio *bio;
@@ -882,9 +880,7 @@ static int ppl_recover_entry(struct ppl_log *log, struct ppl_header_entry *e,
 				 (unsigned long long)r_sector, dd_idx,
 				 (unsigned long long)sector);
 
-			/* Array has not started so rcu dereference is safe */
-			rdev = rcu_dereference_protected(
-					conf->disks[dd_idx].rdev, 1);
+			rdev = conf->disks[dd_idx].rdev;
 			if (!rdev || (!test_bit(In_sync, &rdev->flags) &&
 				      sector >= rdev->recovery_offset)) {
 				pr_debug("%s:%*s data member disk %d missing\n",
@@ -936,9 +932,7 @@ static int ppl_recover_entry(struct ppl_log *log, struct ppl_header_entry *e,
 				0, &disk, &sh);
 		BUG_ON(sh.pd_idx != le32_to_cpu(e->parity_disk));
 
-		/* Array has not started so rcu dereference is safe */
-		parity_rdev = rcu_dereference_protected(
-					conf->disks[sh.pd_idx].rdev, 1);
+		parity_rdev = conf->disks[sh.pd_idx].rdev;
 
 		BUG_ON(parity_rdev->bdev->bd_dev != log->rdev->bdev->bd_dev);
 		pr_debug("%s:%*s write parity at sector %llu, disk %pg\n",
@@ -1404,9 +1398,7 @@ int ppl_init_log(struct r5conf *conf)
 
 	for (i = 0; i < ppl_conf->count; i++) {
 		struct ppl_log *log = &ppl_conf->child_logs[i];
-		/* Array has not started so rcu dereference is safe */
-		struct md_rdev *rdev =
-			rcu_dereference_protected(conf->disks[i].rdev, 1);
+		struct md_rdev *rdev = conf->disks[i].rdev;
 
 		mutex_init(&log->io_mutex);
 		spin_lock_init(&log->io_list_lock);

drivers/md/raid5.h

@@ -473,8 +473,8 @@ enum {
  */
 
 struct disk_info {
-	struct md_rdev	__rcu *rdev;
-	struct md_rdev  __rcu *replacement;
+	struct md_rdev	*rdev;
+	struct md_rdev	*replacement;
 	struct page	*extra_page; /* extra page to use in prexor */
 };