On Fri, Dec 16, 2022 at 08:31:54PM -0600, Linus Torvalds wrote: > Ok, let's bring in Waiman for the rwlock side. > > On Fri, Dec 16, 2022 at 5:54 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote: > > > > Right, for a reader not in_interrupt(), it may be blocked by a random > > waiting writer because of the fairness, even the lock is currently held > > by a reader: > > > > CPU 1 CPU 2 CPU 3 > > read_lock(&tasklist_lock); // get the lock > > > > write_lock_irq(&tasklist_lock); // wait for the lock > > > > read_lock(&tasklist_lock); // cannot get the lock because of the fairness > > But this should be ok - because CPU1 can make progress and eventually > release the lock. > Yes. > So the tasklist_lock use is fine on its own - the reason interrupts > are special is because an interrupt on CPU 1 taking the lock for > reading would deadlock otherwise. As long as it happens on another > CPU, the original CPU should then be able to make progress. > > But the problem here seems to be thst *another* lock is also involved > (in this case apparently "host->lock", and now if CPU1 and CPU2 get > these two locks in a different order, you can get an ABBA deadlock. > Right. > And apparently our lockdep machinery doesn't catch that issue, so it > doesn't get flagged. > I'm confused. Isn't the original problem showing that lockdep catches this? > I'm not sure what the lockdep rules for rwlocks are, but maybe lockdep > treats rwlocks as being _always_ unfair, not knowing about that "it's > only unfair when it's in interrupt context". > The rules nowadays are: * If the reader is in_interrupt() or queued-spinlock implemention is not used, it's an unfair reader, i.e. it won't wait for any existing writer. * Otherwise, it's a fair reader. > Maybe we need to always make rwlock unfair? Possibly only for tasklist_lock? > That's possible, but I need to make sure I understand the issue for lockdep. It's that lockdep misses catching something or it has a false positive? Regards, Boqun > Oh, how I hate tasklist_lock. It's pretty much our one remaining "one > big lock". It's been a pain for a long long time. > > Linus
