Replaying what Tejun wrote: Hello, Oleg.
> Replaying what Oleg wrote: > > Hi, > > Sorry I don't understand the context and I can't find this thread > anywhere, so I am not sure I understand... > Weird, lkml cc is missing. Srivatsa? [Now fixed. This thread has lkml CC] >> Replaying what Tejun wrote: >> So, we basically need percpu_rwlock. We already have percpu_rwsem. > > Yes, and with -mm patches it becomes reader-friendly. In particular > see http://marc.info/?l=linux-mm-commits&m=135240650828875 > >> Oleg, it seems >> CPU hotplug needs big-reader rwlock, ideas on how to proceed? >> > > I am going to convert get_online_cpus() to use percpu_down_read(), > this looks simple. > > We already discussed this with Paul, see > > http://marc.info/?l=linux-kernel&m=135248463226031 > > and the whole thread. > > In short, all we need is percpu_down_write_recursive_readers() and > afaics the only complication is lockdep, we need down_read_no_lockdep() > which (like __up_read) doesn't do rwsem_acquire_read(). > So, it's a different thing. There are two mechanism protecting against cpu hotplug - get_online_cpus() and preempt_disable(). The former can be used by ones which can sleep and need to protect against the whole up/down process (DOWN_PREPARE and so on). The latter protects the last step and can be used when the caller can't sleep. Replacing get_online_cpus() w/ percpu_rwsem is great but this thread is about replacing preempt_disable with something finer grained and less heavy on the writer side - IOW, percpu_rwlock as opposed to percpu_rwsem, so, I think the end result would be that CPU hotplug will be protected by percpu_rwsem for the whole part and by percpu_rwlock for the last commit stage. The problem seems that we don't have percpu_rwlock yet. It shouldn't be too difficult to implement, right? Thanks. -- tejun -- 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/
