On Tue, 2017-04-25 at 16:55 +0200, Peter Rosin wrote: > On 2017-04-25 16:16, Peter Rosin wrote: > > On 2017-04-24 16:59, Philipp Zabel wrote: > >> On Mon, 2017-04-24 at 16:36 +0200, Peter Rosin wrote: > >> [...] > >>>> How about an atomic use_count on the mux_control, a bool shared that is > >>>> only set by the first consumer, and controls whether selecting locks? > >>> > >>> That has the drawback that it is hard to restore the mux-control in a safe > >>> way so that exclusive consumers are allowed after the last shared consumer > >>> puts the mux away. > >> > >> True. > >> > >>> Agreed, it's a corner case, but I had this very similar > >>> patch going through the compiler when I got this mail. Does it work as well > >>> as what you suggested? > >> > >> Yes, this patch works just as well. > > > > Right, as expected :-) However, I don't like it much. It divides the mux > > consumers into two camps in a way that makes it difficult to select which > > camp a consumer should be in. > > > > E.g. consider the iio-mux. The current implementation only supports quick > > accesses that fit the mux_control_get_shared case. But if that mux in the > > future needs to grow continuous buffered accesses, I think there will be > > pressure to switch it over to the exclusive mode. Because that is a lot > > closer to what you are doing with the video-mux. And then what? It will be > > impossible to predict if the end user is going to use buffered accesses or > > not... > > > > So, I think the best approach is to skip the distinction between shared > > and exclusive consumers and instead implement the locking with an ordinary > > semaphore (instead of the old rwsem or the current mutex). Semaphores don't > > have the property that the same task should down/up them (mutexes require > > that for lock/unlock, and is also the reason for the lockdep complaint) and > > thus fits better for long-time use such as yours or the above iio-mux with > > buffered accesses. It should also hopefully be cheaper that an rwsem, and > > not have any downgrade_write calls thus possibly keeping Greg sufficiently > > happy... No idea whether this will placate Greg, but it does work for the video-mux case. The documentation for mux_control_(try_)select should mention that these calls will hold the mux lock until deselect is called, and the documentation for mux_control_select should probably mention that it will block until the lock is released. > > Sure, consumers can still dig themselves into a hole by not calling deselect > > as they should, but at least I think it can be made to work w/o dividing the > > consumers... > > Like this (only compile-tested). Philipp, it should work the same as with > the rwsem in v13 and earlier. At least for your case... regards Philipp -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
