btrfs: make btrfs_discard_workfn() block_group ref explicit

[ Upstream commit 895c6721d310c036dcfebb5ab845822229fa35eb ]

Currently, the async discard machinery owns a ref to the block_group
when the block_group is queued on a discard list. However, to handle
races with discard cancellation and the discard workfn, we have a
specific logic to detect that the block_group is *currently* running in
the workfn, to protect the workfn's usage amidst cancellation.

As far as I can tell, this doesn't have any overt bugs (though
finish_discard_pass() and remove_from_discard_list() racing can have a
surprising outcome for the caller of remove_from_discard_list() in that
it is again added at the end).

But it is needlessly complicated to rely on locking and the nullity of
discard_ctl->block_group. Simplify this significantly by just taking a
refcount while we are in the workfn and unconditionally drop it in both
the remove and workfn paths, regardless of if they race.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/btrfs/discard.c

@@ -152,13 +152,7 @@ static bool remove_from_discard_list(struct btrfs_discard_ctl *discard_ctl,
 	block_group->discard_eligible_time = 0;
 	queued = !list_empty(&block_group->discard_list);
 	list_del_init(&block_group->discard_list);
-	/*
+	if (queued)
-	 * If the block group is currently running in the discard workfn, we
-	 * don't want to deref it, since it's still being used by the workfn.
-	 * The workfn will notice this case and deref the block group when it is
-	 * finished.
-	 */
-	if (queued && !running)
 		btrfs_put_block_group(block_group);
 
 	spin_unlock(&discard_ctl->lock);
@@ -256,9 +250,10 @@ again:
 			block_group->discard_cursor = block_group->start;
 			block_group->discard_state = BTRFS_DISCARD_EXTENTS;
 		}
-		discard_ctl->block_group = block_group;
 	}
 	if (block_group) {
+		btrfs_get_block_group(block_group);
+		discard_ctl->block_group = block_group;
 		*discard_state = block_group->discard_state;
 		*discard_index = block_group->discard_index;
 	}
@@ -482,9 +477,20 @@ static void btrfs_discard_workfn(struct work_struct *work)
 
 	block_group = peek_discard_list(discard_ctl, &discard_state,
 					&discard_index, now);
-	if (!block_group || !btrfs_run_discard_work(discard_ctl))
+	if (!block_group)
 		return;
+	if (!btrfs_run_discard_work(discard_ctl)) {
+		spin_lock(&discard_ctl->lock);
+		btrfs_put_block_group(block_group);
+		discard_ctl->block_group = NULL;
+		spin_unlock(&discard_ctl->lock);
+		return;
+	}
 	if (now < block_group->discard_eligible_time) {
+		spin_lock(&discard_ctl->lock);
+		btrfs_put_block_group(block_group);
+		discard_ctl->block_group = NULL;
+		spin_unlock(&discard_ctl->lock);
 		btrfs_discard_schedule_work(discard_ctl, false);
 		return;
 	}
@@ -536,15 +542,7 @@ static void btrfs_discard_workfn(struct work_struct *work)
 	spin_lock(&discard_ctl->lock);
 	discard_ctl->prev_discard = trimmed;
 	discard_ctl->prev_discard_time = now;
-	/*
+	btrfs_put_block_group(block_group);
-	 * If the block group was removed from the discard list while it was
-	 * running in this workfn, then we didn't deref it, since this function
-	 * still owned that reference. But we set the discard_ctl->block_group
-	 * back to NULL, so we can use that condition to know that now we need
-	 * to deref the block_group.
-	 */
-	if (discard_ctl->block_group == NULL)
-		btrfs_put_block_group(block_group);
 	discard_ctl->block_group = NULL;
 	__btrfs_discard_schedule_work(discard_ctl, now, false);
 	spin_unlock(&discard_ctl->lock);