On Thu, Mar 07, 2019 at 12:48:28AM +0000, Al Viro wrote: > On Thu, Mar 07, 2019 at 12:41:59AM +0000, Al Viro wrote: > > On Wed, Mar 06, 2019 at 04:23:04PM -0800, Linus Torvalds wrote: > > > On Wed, Mar 6, 2019 at 4:03 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > From: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > > > > > > > > "aio: remove the extra get_file/fput pair in io_submit_one" was > > > > too optimistic - not dereferencing file pointer after e.g. > > > > ->write_iter() returns is not enough; that reference might've been > > > > the only thing that kept alive objects that are referenced > > > > *before* the method returns. Such as inode, for example... > > > > > > I still; think that this is actually _worse_ than just having the > > > refcount on the req instead. > > > > > > As it is, we have that completely insane "ref can go away from under > > > us", because nothing keeps that around, which then causes all those > > > other crazy issues with "woken" etc garbage. > > > > > > I think we should be able to get rid of those entirely. Make the > > > poll() case just return zero if it has added the entry successfully to > > > poll queue. No need for "woken", no need for all that odd "oh, but > > > now the req might no longer exist". > > > > Not really. Sure, you can get rid of "might no longer exist" > > considerations, but you still need to decide which way do we want to > > handle it. There are 3 cases: > > * it's already taken up; don't put on the list for possible > > cancel, don't call aio_complete(). > > * will eventually be woken up; put on the list for possible > > cancle, don't call aio_complete(). > > * wanted to be on several queues, fortunately not woken up > > yet. Make sure it's gone from queue, return an error. > > * none of the above, and ->poll() has reported what we wanted > > from the very beginning. Remove from queue, call aio_complete(). > > > > You'll need some logics to handle that. I can buy the "if we know > > the req is still alive, we can check if it's still queued instead of > > separate woken flag", but but it won't win you much ;-/ > > If anything, the one good reason for refcount would be the risk that > some ->read_iter() or ->write_iter() will try to dereference iocb > after having decided to return -EIOCBQUEUED and submitted all bios. > I think that doesn't happen, but making sure it doesn't would be > a good argument in favour of that refcount. *grumble* It is a good argument, unfortunately ;-/ Proof that instances do not step into that is rather subtle and won't take much to break. OK... I'll try to massage that series on top of your patch; I still hate the post-vfs_poll() logics in aio_poll() ;-/ Give me about half an hour and I'll have something to post.
