Messages in this thread | | | Date | Fri, 4 Oct 2013 14:13:00 +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 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.
Oleg.
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