The WARN is great. Something as general as KCSAN isn't necessary: it's a classic ABA problem, double checking the ->bflags value on transitions is sufficient t

The WARN is great. Something as general as KCSAN isn't necessary: it's a classic ABA problem, double checking the ->bflags value on transitions is sufficient to catch it. Like a lockdep-style thing where *_bit() are optionally replaced by helpers that check the current value and WARN if the transition is unexpected. Using the event numbering from your patch description, such a thing would have flagged seeing UPTODATE at (2). But the space of invalid transitions is is much larger than the space of valid ones, which is why I think it might help catch other future bugs sooner. Dunno if it's actually worth it, but I definitely recall bugs of this flavor in the past. It'll take some work to unearth them all, alas...