On Fri, Oct 04, 2013 at 02:13:00PM +0200, Oleg Nesterov wrote: > On 10/04, Peter Zijlstra wrote: > > > > On Fri, Oct 04, 2013 at 01:15:13PM +0200, Oleg Nesterov wrote: > > > > What's exclusive to mean? One writer at a time? > > > > > > Yes, > > > > I'm not entirely sure what the advantage is of having that logic in this > > primitive. Shouldn't that be something the user of this rcu_sync stuff > > does (or not) depending on its needs. > > Yes, the user can do the locking itself. But I think this option can help. > If nothing else it can help to avoid another mutex/whatever and unnecessary > wakeup/scheule's, even if this is minor. > > And. rcu_sync_enter() should be "bool", it should return "need_sync". IOW, > rcu_sync_enter() == T means that this thread has done the FAST -> SLOW > transition, this is particularly useful in "exclusive" mode. > > Consider percpu_down_write(). It takes rw_sem for writing (and this blocks > the readers) before clear_fast_ctr(), but we only need to do this this > after sync_sched(), so it could do > > if (rcu_sync_enter(&brw->rcu_sync)) > atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr); > else > ; /* the above was already done */ > > /* exclude readers */ > down_write(&brw->rw_sem); > > and now ->rw_sem is only needed to serialize readers/writer. > > Sure, this all is minor (and we will probably copy the "pending writer" > logic from cpu_hotplug_begin/get_online_cpus). > > But we can get this feature almost for free, so I think it makes sense.
Well, the whole reason I asked is because adding that completion in there didn't at all smell like free to me; not to mention that I hadn't at all realized you're using it as a semaphore. Also; what would be the use once you convert the per-cpu rwsem over to the scheme I used with hotplug? I'm really starting to think we shouldn't do this in rcu_sync at all. -- 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/
