On Mon, Oct 14, 2013 at 08:23:03AM -0700, Frank Filz wrote: > > http://www.samba.org/samba/news/articles/low_point/tale_two_stds_os2 > > > > .html > > > > > > > > See the section entitled "First Implementation Past the Post". > > > > > > Interesting that Jeremy actually suggested the implementation should > > > have had an arbitrary lock owner as part of the flock structure: > > > > > > "This is an example of a POSIX interface not being future-proofed > > > against modern techniques such as threading. A simple amendment to the > > > original primitive allowing a user-defined "locking context" (like a > > > process id) to be entered in the struct flock structure used to define > > > the lock would have fixed this problem, along with extra flags > > > allowing the number of locks per context to be recorded if needed." > > > > > > But I'm happy with the lock context per kernel struct file as a > > > solution, especially since that will allow locks to be sensibly passed > > > to a forked process. > > > > > > Another next step would be an asynchronous blocking lock... > > > > Yes, please :-) > > What model would be useful to you (and for what project)? One thing I could It's ctdb that would be mainly interested in this. ctdb deals a lot with out tdb files, a shared mmap key/value database protected by fcntl locks. ctdb is the database daemon distributing records in a cluster. It is a single-threaded async event loop, and it has to fork helper processes waiting for locks. > think of is since we have a file descriptor for each lock owner/file pair, > we could do something like select on those descriptors, got to think about > how that would actually work though... The vfs lock layer does inherently > support a kernel call back when a blocked lock can be unblocked, so we just > need to figure out the best way to hook that up to user space in a way that > doesn't require a thread per blocked lock. A model that would probably work for us is one file descriptor that becomes readable when one of the blocking lock states changes. To signal which one changed, I think passing an opaque uint64 (usable as a pointer) for the blocking lock would be great, or possibly something like epoll_data_t. We would pass this in the fcntl call and read it from the signal, possibly together with an errno (EDEADLK?). Not sure if this is feasible kernel-side, but I believe this is something that would work for us user-side. Volker -- SerNet GmbH, Bahnhofsallee 1b, 37081 Göttingen phone: +49-551-370000-0, fax: +49-551-370000-9 AG Göttingen, HRB 2816, GF: Dr. Johannes Loxen http://www.sernet.de, mailto:kontakt@xxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
