On Tue, Mar 12, 2013 at 9:47 AM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: > On Mon, 2013-03-11 at 19:28 -0700, Michel Lespinasse wrote: >> Also why the write-priority requirement rather than reader-writer >> fairness ? Is it to make it less likely to hit the writer timeouts ? > > Since tty i/o can be really [painfully] slow, allowing waiting future > references to succeed is not an option. All right, that makes sense after your explanation. > I understand the concern regarding the potential proliferation of new > lock types. Lock implementations are hard to get right, and no one wants > to debug 7 different lock policy implementations of a read/write > semaphore. > > OTOH, 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. Agree that having a suitable lock for your usage is much nicer than having ad-hoc solutions. > 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. Have you considered building your ldlock based on lib/rwsem-spinlock.c instead ? i.e. having an internal spinlock to protect the ldisc reference count and the reader and writer queues. This would seem much simpler get right. The downside would be that a spinlock would be taken for a short time whenever an ldisc reference is taken or released. I don't expect that the internal spinlock would get significant contention ? > Perhaps a future direction for rwsem would be to provide a selectable > lock policy (fifo, mostly-fair, writer-first) on initialization so that > the different use cases can be easily accomodated? Probably makes more sense to have different locks for the different usage models IMO... -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- 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
