On Mon, Mar 18, 2013 at 09:01:19PM -0400, Peter Hurley wrote: > On Mon, 2013-03-18 at 16:58 -0700, Greg Kroah-Hartman wrote: > > On Mon, Mar 11, 2013 at 04:44:46PM -0400, Peter Hurley wrote: > > > 2) TIOCSETD ioctl (change line discipline) expects to return an > > > error if the line discipline cannot be exclusively locked within > > > 5 secs. Lock wait timeouts are not supported by rwsem. > > > > Don't we have some other lock that can timeout? > > Not that behaves like a r/w semaphore. Can't we just add it? Or is that too much work? > > > 3) A tty hangup is expected to halt and scrap pending i/o, so > > > exclusive locking must be prioritized without precluding > > > existing reference holders from obtaining recursive read locks. > > > Writer priority is not supported by rwsem. > > > > But how bad is it really if we have to wait a bit for that write lock to > > get through all of the existing readers? Either way, we are supposed to > > be dropping i/o, so it shouldn't be a big deal, right? > > The rwsem behavior is in the process of changing. Write lock stealing > has already been added and refinements there will likely allow some > readers in front of writers. > > With slow serial i/o, I'd rather have hangups occur promptly than let a > bunch more i/o through. So all we are now lacking, with the changes to rwsem, is the timeout problem? > > > Add ld_semaphore which implements these requirements in a > > > semantically and operationally similar way to rw_semaphore. > > > > I _really_ don't want to add a new lock to the kernel, especially one > > that is only used by one "driver". You are going to have to convince > > the current lock authors that this really is needed, before I can take > > it, sorry. > > That's fine. I can understand the reluctance to take on a new lock > [although you might be interested to read my analysis of rwsem here > https://lkml.org/lkml/2013/3/11/533 which outlines an existing flaw]. > > That said, part of the reason why the current ldisc implementation is > broken is the lack of appropriate locks. As I recently explained > (actually in this patchset's thread), > > a lack of existing options has spawned a DIY approach without > higher-order locks that is rarely correct, but which goes largely > unnoticed exactly because it's not a new lock. A brief review of the > hangs, races, and deadlocks fixed by this patchset should be convincing > enough of that fact. In my opinion, this is the overriding concern. > > The two main problems with a one-size-fits-all lock policy is that, > 1) lock experts can't realistically foresee the consequences of policy > changes without already being experts in the subsystems in which that > lock is used. Even domain experts may miss potential consequences, and > 2) domain experts typically wouldn't even consider writing a new lock. > So they make do with atomic bit states, spinlocks, reference counts, > mutexes, and waitqueues, making a mostly-functional, higher-order lock. I read that, however rolling your own lock is almost never the solution. > From whom would you like me to get an ack for this? The people who wrote the rwsem code? > > What is wrong with the existing ldisc code that the creation of this > > lock is needed? Is our current code that broken? > > Yes. Even just the acquistion of the ldisc reference is wrong [the > analysis is in the patch 21 changelog]. Yes, very nice work, I'm not saying that this isn't a messed up area at all, it's just that such deep flaws that require a new type of a lock don't usually come up all that often. > If you'd like, I can send you 6 or so short user test programs that > hang, crash, or deadlock inside 60 seconds on mainline and next, but not > with this patchset. That would be interesting to have, please send them. And I hope that they only lock up when run as root, but I'm afraid to ask that question... thanks, greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
