A background thread performing blocking io is an implementation detail not a bug. Other filesystems don’t have/need that sort of bookkeeping, so if a block device stalls badly enough to trigger these warnings then it will be attributed to application threads (if at all) rather than btrfs worker threads, but regardless the stall very much still happens

> if a block device stalls badly

That's really the issue at heart, because I've seen these on zfs as well... but you'd think the filesystem would report some progress to keep bumping the timer so it doesn't start spamming dmesg. /shrug