On Mon, 2013-03-18 at 18:59 -0700, Greg Kroah-Hartman wrote: > 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? See my comments below about rolling your own lock. > > > > 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? No. What I'm saying is the existing rwsem lock policy is not ideal and the future lock policy is unlikely to be any more ideal, and possibly worse. > > > > 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. Except that's the whole issue, isn't it? There is no existing lock solution because there is no r/w semaphore that times out. So, no matter what, a new lock is required. Since there is only one use-case for the new lock, it makes sense for that lock to have the lock policy best suited for the one use-case, especially since it's already done. > > From whom would you like me to get an ack for this? > > The people who wrote the rwsem code? Ok. Regards, Peter Hurley -- 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
