Hi, On Sat, Jun 3, 2023 at 6:03 AM Andreas Gruenbacher <agruenba@xxxxxxxxxx> wrote: > > On Thu, Jun 1, 2023 at 9:10 PM Alexander Aring <aahringo@xxxxxxxxxx> wrote: > > Hi, > > > > On Thu, Jun 1, 2023 at 1:11 PM Andreas Gruenbacher <agruenba@xxxxxxxxxx> wrote: > > > > > > On Thu, Jun 1, 2023 at 6:28 PM Alexander Aring <aahringo@xxxxxxxxxx> wrote: > > > > Hi, > > > > > > > > On Tue, May 30, 2023 at 1:40 PM Andreas Gruenbacher <agruenba@xxxxxxxxxx> wrote: > > > > > > > > > > On Tue, May 30, 2023 at 4:08 PM Alexander Aring <aahringo@xxxxxxxxxx> wrote: > > > > > > Hi, > > > > > > > > > > > > On Tue, May 30, 2023 at 7:01 AM Andreas Gruenbacher <agruenba@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > On Tue, May 30, 2023 at 12:19 AM Alexander Aring <aahringo@xxxxxxxxxx> wrote: > > > > > > > > Hi, > > > > > > > > > > > > > > > > On Thu, May 25, 2023 at 11:02 AM Andreas Gruenbacher > > > > > > > > <agruenba@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > On Wed, May 24, 2023 at 6:02 PM Alexander Aring <aahringo@xxxxxxxxxx> wrote: > > > > > > > > > > This patch fixes a possible plock op collisions when using F_SETLKW lock > > > > > > > > > > requests and fsid, number and owner are not enough to identify a result > > > > > > > > > > for a pending request. The ltp testcases [0] and [1] are examples when > > > > > > > > > > this is not enough in case of using classic posix locks with threads and > > > > > > > > > > open filedescriptor posix locks. > > > > > > > > > > > > > > > > > > > > The idea to fix the issue here is to place all lock request in order. In > > > > > > > > > > case of non F_SETLKW lock request (indicated if wait is set or not) the > > > > > > > > > > lock requests are ordered inside the recv_list. If a result comes back > > > > > > > > > > the right plock op can be found by the first plock_op in recv_list which > > > > > > > > > > has not info.wait set. This can be done only by non F_SETLKW plock ops as > > > > > > > > > > dlm_controld always reads a specific plock op (list_move_tail() from > > > > > > > > > > send_list to recv_mlist) and write the result immediately back. > > > > > > > > > > > > > > > > > > > > This behaviour is for F_SETLKW not possible as multiple waiters can be > > > > > > > > > > get a result back in an random order. To avoid a collisions in cases > > > > > > > > > > like [0] or [1] this patch adds more fields to compare the plock > > > > > > > > > > operations as the lock request is the same. This is also being made in > > > > > > > > > > NFS to find an result for an asynchronous F_SETLKW lock request [2][3]. We > > > > > > > > > > still can't find the exact lock request for a specific result if the > > > > > > > > > > lock request is the same, but if this is the case we don't care the > > > > > > > > > > order how the identical lock requests get their result back to grant the > > > > > > > > > > lock. > > > > > > > > > > > > > > > > > > When the recv_list contains multiple indistinguishable requests, this > > > > > > > > > can only be because they originated from multiple threads of the same > > > > > > > > > process. In that case, I agree that it doesn't matter which of those > > > > > > > > > requests we "complete" in dev_write() as long as we only complete one > > > > > > > > > request. We do need to compare the additional request fields in > > > > > > > > > dev_write() to find a suitable request, so that makes sense as well. > > > > > > > > > We need to compare all of the fields that identify a request (optype, > > > > > > > > > ex, wait, pid, nodeid, fsid, number, start, end, owner) to find the > > > > > > > > > "right" request (or in case there is more than one identical request, > > > > > > > > > a "suitable" request). > > > > > > > > > > > > > > > > > > > > > > > > > In my "definition" why this works is as you said the "identical > > > > > > > > request". There is a more deeper definition of "when is a request > > > > > > > > identical" and in my opinion it is here as: "A request A is identical > > > > > > > > to request B when they get granted under the same 'time'" which is all > > > > > > > > the fields you mentioned. > > > > > > > > > > > > > > > > Even with cancellation (F_SETLKW only) it does not matter which > > > > > > > > "identical request" you cancel because the kernel and user > > > > > > > > (dlm_controld) makes no relation between a lock request instance. You > > > > > > > > need to have at least the same amount of "results" coming back from > > > > > > > > user space as the amount you are waiting for a result for the same > > > > > > > > "identical request". > > > > > > > > > > > > > > That's not incorrect per se, but cancellations create an additional > > > > > > > difficulty: they can either succeed or fail. To indicate that a > > > > > > > cancellation has succeeded, a new type of message can be introduced > > > > > > > (say, "CANCELLED"), and it's obvious that a CANCELLED message can only > > > > > > > belong to a locking request that is being cancelled. When cancelling a > > > > > > > locking request fails, the kernel will see a "locking request granted" > > > > > > > message though, and when multiple identical locking requests are > > > > > > > queued and only some of them have been cancelled, it won't be obvious > > > > > > > which locking request a "locking request granted" message should be > > > > > > > assigned to anymore. You really don't want to mix things up in that > > > > > > > case. > > > > > > > > > > > > > > This complication makes it a whole lot more difficult to reason about > > > > > > > the correctness of the code. All that complexity is avoidable by > > > > > > > sticking with a fixed mapping of requests and replies (i.e., a unique > > > > > > > request identifier). > > > > > > > > > > > > > > To put it differently, you can shoot yourself in the foot and still > > > > > > > hop along on the other leg, but it may not be the best of all possible > > > > > > > ideas. > > > > > > > > > > > > > > > > > > > It makes things more complicated, I agree and the reason why this > > > > > > works now is because there are a lot of "dependencies". I would love > > > > > > to have an unique identifier to make it possible that we can follow an > > > > > > instance handle of the original lock request. > > > > > > > > > > > > * an unique identifier which also works with the async lock request of > > > > > > lockd case. > > > > > > > > > > What's the lockd case you're referring to here, and why is it relevant > > > > > for the problem at hand? > > > > > > > > just mentioned that we need a solution which also works for the > > > > asynchronous lock request (F_SETLK, F_SETLKW) case, there is only one > > > > user lockd. [0] DLM plock handling implements the behaviour mentioned > > > > at [0] but lm_grant() callback can also return negative values and > > > > signals that the lock request was cancelled (on nfs layer) and then > > > > need to tell it DLM. This however is not supported as we have a lack > > > > of cancellation. > > > > > > Ouch, that's a bit messy. But if the vfs_lock_file() description is > > > accurate, then only F_SETLK requests arriving via lockd can be > > > asynchronous, and F_SETLKW requests never are asynchronous. And we > > > only need to cancel F_SETLKW requests. It follows that we only ever > > > need to cancel synchronous requests. > > > > > > > I looked into the code and I think the second sentence of [0] is > > important regarding to F_SLEEP "Callers expecting ->lock() to return > > asynchronously will only use F_SETLK, not F_SETLKW; they will set > > FL_SLEEP if (and only if) the request is for a blocking lock.". > > If lockd does a lock request, it will then set args.wait to 1 if it > > was F_SETLKW [1]. > > Hmm, this sets args.block? > You are right but args.block gets passed as the wait parameter to nlmsvc_lock(). An example user: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/lockd/svc4proc.c#n158 > > The receiver will then create a blocking request [2] > > and set FL_SLEEP [3]; it does unset FL_SLEEP when args.wait (now as > > wait parameter) wasn't set. There is a case of [5] which unset wait > > again, but it seems we can end with FL_SLEEP there anyway. > > > > I think we have currently an issue here that we handle F_SETLK when > > F_SLEEP (in case of async lockd request) is handled as trylock which > > isn't right. Don't ask me why they are going over F_SLEEP and F_SETLK > > and not just using F_SETLKW to signal that it is a blocking request. > > So we actually have the F_SETLKW case but it's just signaled with > > F_SETLK + FL_SLEEP? > > It seems to me that the vfs_lock_file() description and the > distinction it makes between F_SETLK and F_SETLKW makes sense in a > scenario when locking is implemented locally in the kernel, but not in > the context of dlm_controld, where all the locking decisions are made > in user-space: in the former scenario, F_SETLK requests can be decided > immediately without sleeping, but F_SETLKW requests may sleep. In the > dlm_controld scenario, locking requests can never be decided > immediately, and the requesting task will always sleep. So from the > point of view of lockd, any request to dlm_controld should probably be > treated as asynchronous. > > This makes things a bit more ugly than I was hoping for. > I think they describe a "blocking" request as what you describe with F_SETLKW. Everything is asynchronous, but F_SETLKW is the "blocking request" by not actually meaning the lock() callback blocks in the sense of wait_event(). There is however a semantic difference between F_SETLK and F_SETLKW in sense when the asynchronous result comes back and what the result is. I looked more into this and a "somewhat" recent commit confuses me more. It's 40595cdc93ed ("nfs: block notification on fs with its own ->lock") [0] and from what I understood, they signaled before F_SETLKW with FL_SLEEP set, but they realized that mostly everybody does not follow this API, so they switch to only F_SETLK and assume that the most nfs clients do polling for a look. If you can't use F_SETLKW, then the alternative could be polling the lock with F_SETLK, which isn't a good design to use on DLM. In the commit message it's also mentioned that they may bring back the old behaviour. I think the comment in [1] is not up to date anymore. There are also only two users of "fl_lmops" evaluation, which is fcntl core and DLM, fuse will reject any lock request when "fl_lmops" is set. To be honest I have no idea how to proceed here, probably just look in fcntl core and do what they do. I also tested dlm/next with a nfsv3 server on gfs2 by running "stress-ng --fcntl 16", I see some worried error messages on the kernel log coming up. - Alex [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/fs/lockd/svclock.c?h=v6.4-rc4&id=40595cdc93edf4110c0f0c0b06f8d82008f23929 [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/locks.c?h=v6.4-rc4#n2255
