Hello, Srivatsa. On Thu, Jan 24, 2013 at 01:03:52AM +0530, Srivatsa S. Bhat wrote: > Hmm.. I split it up into steps to help explain the reasoning behind > the code sufficiently, rather than spring all of the intricacies at > one go (which would make it very hard to write the changelog/comments > also). The split made it easier for me to document it well in the > changelog, because I could deal with reasonable chunks of code/complexity > at a time. IMHO that helps people reading it for the first time to > understand the logic easily. I don't know. It's a judgement call I guess. I personally would much prefer having ample documentation as comments in the source itself or as a separate Documentation/ file as that's what most people are gonna be looking at to figure out what's going on. Maybe just compact it a bit and add more in-line documentation instead? > > The only two options are either punishing writers or identifying and > > updating all such possible deadlocks. percpu_rwsem does the former, > > right? I don't know how feasible the latter would be. > > I don't think we can avoid looking into all the possible deadlocks, > as long as we use rwlocks inside get/put_online_cpus_atomic() (assuming > rwlocks are fair). Even with Oleg's idea of using synchronize_sched() > at the writer, we still need to take care of locking rules, because the > synchronize_sched() only helps avoid the memory barriers at the reader, > and doesn't help get rid of the rwlocks themselves. Well, percpu_rwlock don't have to use rwlock for the slow path. It can implement its own writer starving locking scheme. It's not like implementing slow path global rwlock logic is difficult. > CPU 0 CPU 1 > > read_lock(&rwlock) > > write_lock(&rwlock) //spins, because CPU 0 > //has acquired the lock for read > > read_lock(&rwlock) > ^^^^^ > What happens here? Does CPU 0 start spinning (and hence deadlock) or will > it continue realizing that it already holds the rwlock for read? I don't think rwlock allows nesting write lock inside read lock. read_lock(); write_lock() will always deadlock. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
