On 12/10, Srivatsa S. Bhat wrote: > > On 12/10/2012 02:27 AM, Oleg Nesterov wrote: > > On 12/07, Srivatsa S. Bhat wrote: > >> > >> 4. No deadlock possibilities > >> > >> Per-cpu locking is not the way to go if we want to have relaxed rules > >> for lock-ordering. Because, we can end up in circular-locking > >> dependencies > >> as explained in https://lkml.org/lkml/2012/12/6/290 > > > > OK, but this assumes that, contrary to what Steven said, read-write-read > > deadlock is not possible when it comes to rwlock_t. > > What I meant is, with a single (global) rwlock, you can't deadlock like that.
Ah. I greatly misunderstood Steven's email, http://marc.info/?l=linux-pm&m=135482212307876 Somehow I didn't notice he described the deadlock with _two_ rwlock's, I wrongly thought that his point is that read_lock() is not recursive (like down_read). > Let me know if my assumptions are incorrect! No, sorry, I misunderstood Steven. > > However. If this is true, then compared to preempt_disable/stop_machine > > livelock is possible. Probably this is fine, we have the same problem with > > get_online_cpus(). But if we can accept this fact I feel we can simmplify > > this somehow... Can't prove, only feel ;) > > Not sure I follow.. I meant that write_lock_irqsave(&hotplug_rwlock) in take_cpu_down() can spin "forever". Suppose that reader_acked() == T on every CPU, so that get_online_cpus_atomic() always takes read_lock(&hotplug_rwlock). It is possible that this lock will be never released by readers, CPU_0 CPU_1 get_online_cpus_atomic() get_online_cpus_atomic() put_online_cpus_atomic() get_online_cpus_atomic() put_online_cpus_atomic() get_online_cpus_atomic() put_online_cpus_atomic() and so on. > Reader-side: > -> read_lock() your per-cpu rwlock and proceed. > > Writer-side: > -> for_each_online_cpu(cpu) > write_lock(per-cpu rwlock of 'cpu'); Yes, yes, this is clear. > Also, like Tejun said, one of the important measures for per-cpu rwlocks > should be that, if a user replaces global rwlocks with percpu rwlocks (for > performance reasons), he shouldn't suddenly end up in numerous deadlock > possibilities which never existed before. The replacement should continue to > remain safe, and perhaps improve the performance. Sure, I agree. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
