On Wed, Jun 28, 2023 at 05:14:09PM -0600, Jens Axboe wrote: > On 6/28/23 4:55?PM, Kent Overstreet wrote: > >> But it's not aio (or io_uring or whatever), it's simply the fact that > >> doing an fput() from an exiting task (for example) will end up being > >> done async. And hence waiting for task exits is NOT enough to ensure > >> that all file references have been released. > >> > >> Since there are a variety of other reasons why a mount may be pinned and > >> fail to umount, perhaps it's worth considering that changing this > >> behavior won't buy us that much. Especially since it's been around for > >> more than 10 years: > > > > Because it seems that before io_uring the race was quite a bit harder to > > hit - I only started seeing it when things started switching over to > > io_uring. generic/388 used to pass reliably for me (pre backpointers), > > now it doesn't. > > I literally just pasted a script that hits it in one second with aio. So > maybe generic/388 doesn't hit it as easily, but it's surely TRIVIAL to > hit with aio. As demonstrated. The io_uring is not hard to bring into > parity on that front, here's one I posted earlier today for 6.5: > > https://lore.kernel.org/io-uring/20230628170953.952923-4-axboe@xxxxxxxxx/ > > Doesn't change the fact that you can easily hit this with io_uring or > aio, and probably more things too (didn't look any further). Is it a > realistic thing outside of funky tests? Probably not really, or at least > if those guys hit it they'd probably have the work-around hack in place > in their script already. > > But the fact is that it's been around for a decade. It's somehow a lot > easier to hit with bcachefs than XFS, which may just be because the > former has a bunch of workers and this may be deferring the delayed fput > work more. Just hand waving. Not sure what you're arguing here...? We've had a long standing bug, it's recently become much easier to hit (for multiple reasons); we seem to be in agreement on all that. All I'm saying is that the existence of that bug previously is not reason to fix it now. > >> then we'd probably want to move that deferred fput list to the > >> task_struct and ensure that it gets run if the task exits rather than > >> have a global deferred list. Currently we have: > >> > >> > >> 1) If kthread or in interrupt > >> 1a) add to global fput list > >> 2) task_work_add if not. If that fails, goto 1a. > >> > >> which would then become: > >> > >> 1) If kthread or in interrupt > >> 1a) add to global fput list > >> 2) task_work_add if not. If that fails, we know task is existing. add to > >> per-task defer list to be run at a convenient time before task has > >> exited. > > > > no, it becomes: > > if we're running in a user task, or if we're doing an operation on > > behalf of a user task, add to the user task's deferred list: otherwise > > add to global deferred list. > > And how would the "on behalf of a user task" work in terms of being > in_interrupt()? I don't see any relation to in_interrupt? We'd have to add a version of fput() that takes an additional task_struct argument, and plumb that through the aio code - kioctx lifetime is tied to mm_struct, not task_struct, so we'd have to add a ref to the task_struct to kiocb. Which would probably be a good thing tbh, it'd let us e.g. account cpu time back to the original task when kiocb completion has to run out of a workqueue.
