Messages in this thread | | | Date | Fri, 4 Oct 2013 15:31:35 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/3] rcu: Create rcu_sync infrastructure |
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On 10/04, Peter Zijlstra wrote: > > On Fri, Oct 04, 2013 at 02:13:00PM +0200, Oleg Nesterov wrote: > > On 10/04, Peter Zijlstra wrote: > > > > > > 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;
Why? this only adds sizeof(long).
If you dislike the idea to add the new __complete_locked() one-liner, this is not strictly necessary, just a bit simpler/understandable.
> not to mention that I hadn't > at all realized you're using it as a semaphore.
And the logic is trivial.
> Also; what would be the use once you convert the per-cpu rwsem over to > the scheme I used with hotplug?
It will still use the exclusive more to block other writers? This avoids another mutex simplifies the code.
> I'm really starting to think we shouldn't do this in rcu_sync at all.
I do not really understand why you insist that rcu_sync() should not try to help to the users which need the exclusive mode.
rcu_sync_enter/exit have to do some work to serialize with each other anyway, we already have ->nr_writers, so why we can't add 3 simple "if exclusive" checks?
Oleg.
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