On Tue, Jan 14, 2014 at 1:51 PM, J. Bruce Fields <bfie...@fieldses.org> wrote: > On Tue, Jan 14, 2014 at 01:26:26PM -0800, Andy Lutomirski wrote: >> [grr, gmail -- I didn't actually intend to send that.] >> >> On Tue, Jan 14, 2014 at 1:24 PM, Andy Lutomirski <l...@amacapital.net> wrote: >> > On Tue, Jan 14, 2014 at 1:19 PM, Frank Filz <ffilz...@mindspring.com> >> > wrote: >> >>> process 2 requests a write lock, gets -EDEADLK, unlocks and >> >>> requests a new read lock. That request succeeds because there >> >>> is no conflicting lock. (Note the lock manager had no >> >>> opportunity to upgrade 1's lock here thanks to the conflict with >> >>> 3's lock.) >> >> >> >> As I understand write lock priority, process 2 requesting a new read lock >> >> would block, once there is a write lock waiter, no further read locks >> >> would >> >> be granted that would conflict with that waiting write lock. >> > >> > ...which reminds me -- if anyone implements writer priority, please >> > make it optional (either w/ a writer-priority-ignoring read lock or a >> > non-priority-granting write lock). I have an application for which >> > writer priority would be really annoying. >> > >> > Even better: Have read-lock-and-wait-for-pending-writers be an explicit >> > new operation. >> > >> > (Writer priority a >> >> Writer priority can introduce new deadlocks. Suppose that a reader >> (holding a read lock) starts a subprocess that takes a new read lock >> and waits for that subprocess. Throw an unrelated process in that >> tries to take a write lock and you have an instant deadlock. > > OK, so we definitely can't silently change existing lock behavior to > prioritize writes in this way. > > A remaining interesting question is whether we'd like the new locks to > support either behavior or both. > > I'd still be inclined to stick to the existing (unprioritized) behavior > just to minimize the scope of the project.
I think it would be silly to change the behavior at all (other than probably documenting that -EDEADLK is a valid return value) until this stuff is merged. None of this has identified anything that's either wrong or unnecessarily limiting about the current proposal, so I see no reason to try to do anything fancy right now. Long term, I'd advocate for a new l_type value F_RDLCK_WAIT_FOR_WRITERS (or the equivalent with a better name) and implementing -EDEADLK, for the case where two overlapping upgrade attempts conflict. If it's indeed true that a failed F_SETLK (or F_SETLKW) does not change lock state, documenting that would be nice, too. Finally, on a completely unrelated note, IIRC lock positions are treated as *signed* integers and can't be negative. Documenting that (or the reverse) would be nice, too. This bit me once, and it's probably briefly confused other people, too. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
