On Mon, Feb 18, 2013 at 8:39 PM, Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote: > Some important design requirements and considerations: > ----------------------------------------------------- > > 1. Scalable synchronization at the reader-side, especially in the fast-path > > Any synchronization at the atomic hotplug readers side must be highly > scalable - avoid global single-holder locks/counters etc. Because, these > paths currently use the extremely fast preempt_disable(); our replacement > to preempt_disable() should not become ridiculously costly and also should > not serialize the readers among themselves needlessly. > > At a minimum, the new APIs must be extremely fast at the reader side > atleast in the fast-path, when no CPU offline writers are active. > > 2. preempt_disable() was recursive. The replacement should also be recursive. > > 3. No (new) lock-ordering restrictions > > preempt_disable() was super-flexible. It didn't impose any ordering > restrictions or rules for nesting. Our replacement should also be equally > flexible and usable. > > 4. No deadlock possibilities > > Regular 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 > > So, avoid the usual per-cpu locking schemes (per-cpu locks/per-cpu atomic > counters with spin-on-contention etc) as much as possible, to avoid > numerous deadlock possibilities from creeping in. > > > Implementation of the design: > ---------------------------- > > We use per-CPU reader-writer locks for synchronization because: > > a. They are quite fast and scalable in the fast-path (when no writers are > active), since they use fast per-cpu counters in those paths. > > b. They are recursive at the reader side. > > c. They provide a good amount of safety against deadlocks; they don't > spring new deadlock possibilities on us from out of nowhere. As a > result, they have relaxed locking rules and are quite flexible, and > thus are best suited for replacing usages of preempt_disable() or > local_irq_disable() at the reader side. > > Together, these satisfy all the requirements mentioned above. Thanks for this detailed design explanation. > +/* > + * Invoked by atomic hotplug reader (a task which wants to prevent > + * CPU offline, but which can't afford to sleep), to prevent CPUs from > + * going offline. So, you can call this function from atomic contexts > + * (including interrupt handlers). > + * > + * Note: This does NOT prevent CPUs from coming online! It only prevents > + * CPUs from going offline. > + * > + * You can call this function recursively. > + * > + * Returns with preemption disabled (but interrupts remain as they are; > + * they are not disabled). > + */ > +void get_online_cpus_atomic(void) > +{ > + percpu_read_lock_irqsafe(&hotplug_pcpu_rwlock); > +} > +EXPORT_SYMBOL_GPL(get_online_cpus_atomic); > + > +void put_online_cpus_atomic(void) > +{ > + percpu_read_unlock_irqsafe(&hotplug_pcpu_rwlock); > +} > +EXPORT_SYMBOL_GPL(put_online_cpus_atomic); So, you made it clear why you want a recursive read side here. I am wondering though, if you could take care of recursive uses in get/put_online_cpus_atomic() instead of doing it as a property of your rwlock: get_online_cpus_atomic() { unsigned long flags; local_irq_save(flags); if (this_cpu_inc_return(hotplug_recusion_count) == 1) percpu_read_lock_irqsafe(&hotplug_pcpu_rwlock); local_irq_restore(flags); } Once again, the idea there is to avoid baking the reader side recursive properties into your rwlock itself, so that it won't be impossible to implement reader/writer fairness into your rwlock in the future (which may be not be very important for the hotplug use, but could be when other uses get introduced). -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
