Messages in this thread | | | Date | Sun, 4 Nov 2018 19:43:30 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [RFC] doc: rcu: remove note on smp_mb during synchronize_rcu |
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On Sat, Nov 03, 2018 at 08:49:56PM -0700, Joel Fernandes wrote: > On Sat, Nov 03, 2018 at 04:22:59PM -0700, Paul E. McKenney wrote: > > On Fri, Nov 02, 2018 at 10:12:26PM -0700, Joel Fernandes wrote: > > > On Thu, Nov 01, 2018 at 09:13:07AM -0700, Paul E. McKenney wrote: > > > > On Wed, Oct 31, 2018 at 10:00:19PM -0700, Joel Fernandes wrote: > > > > > On Wed, Oct 31, 2018 at 11:17:48AM -0700, Paul E. McKenney wrote: > > > > > > On Tue, Oct 30, 2018 at 06:11:19PM -0700, Joel Fernandes wrote: > > > > > > > Hi Paul, > > > > > > > > > > > > > > On Tue, Oct 30, 2018 at 04:43:36PM -0700, Paul E. McKenney wrote: > > > > > > > > On Tue, Oct 30, 2018 at 03:26:49PM -0700, Joel Fernandes wrote: > > > > > > > > > Hi Paul, > > > > > > > > > > > > > > > > > > On Sat, Oct 27, 2018 at 09:30:46PM -0700, Joel Fernandes (Google) wrote: > > > > > > > > > > As per this thread [1], it seems this smp_mb isn't needed anymore: > > > > > > > > > > "So the smp_mb() that I was trying to add doesn't need to be there." > > > > > > > > > > > > > > > > > > > > So let us remove this part from the memory ordering documentation. > > > > > > > > > > > > > > > > > > > > [1] https://lkml.org/lkml/2017/10/6/707 > > > > > > > > > > > > > > > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > > > > > > > > > > > > > > > > > > I was just checking about this patch. Do you feel it is correct to remove > > > > > > > > > this part from the docs? Are you satisified that a barrier isn't needed there > > > > > > > > > now? Or did I miss something? > > > > > > > > > > > > > > > > Apologies, it got lost in the shuffle. I have now applied it with a > > > > > > > > bit of rework to the commit log, thank you! > > > > > > > > > > > > > > No worries, thanks for taking it! > > > > > > > > > > > > > > Just wanted to update you on my progress reading/correcting the docs. The > > > > > > > 'Memory Ordering' is taking a bit of time so I paused that and I'm focusing > > > > > > > on finishing all the other low hanging fruit. This activity is mostly during > > > > > > > night hours after the baby is asleep but some times I also manage to sneak it > > > > > > > into the day job ;-) > > > > > > > > > > > > If there is anything I can do to make this a more sustainable task for > > > > > > you, please do not keep it a secret!!! > > > > > > > > > > Thanks a lot, that means a lot to me! Will do! > > > > > > > > > > > > BTW I do want to discuss about this smp_mb patch above with you at LPC if you > > > > > > > had time, even though we are removing it from the documentation. I thought > > > > > > > about it a few times, and I was not able to fully appreciate the need for the > > > > > > > barrier (that is even assuming that complete() etc did not do the right > > > > > > > thing). Specifically I was wondering same thing Peter said in the above > > > > > > > thread I think that - if that rcu_read_unlock() triggered all the spin > > > > > > > locking up the tree of nodes, then why is that locking not sufficient to > > > > > > > prevent reads from the read-side section from bleeding out? That would > > > > > > > prevent the reader that just unlocked from seeing anything that happens > > > > > > > _after_ the synchronize_rcu. > > > > > > > > > > > > Actually, I recall an smp_mb() being added, but am not seeing it anywhere > > > > > > relevant to wait_for_completion(). So I might need to add the smp_mb() > > > > > > to synchronize_rcu() and remove the patch (retaining the typo fix). :-/ > > > > > > > > > > No problem, I'm glad atleast the patch resurfaced the topic of the potential > > > > > issue :-) > > > > > > > > And an smp_mb() is needed in Tree RCU's __wait_rcu_gp(). This is > > > > because wait_for_completion() might get a "fly-by" wakeup, which would > > > > mean no ordering for code naively thinking that it was ordered after a > > > > grace period. > > > > > > > > > > The short form answer is that anything before a grace period on any CPU > > > > > > must be seen by any CPU as being before anything on any CPU after that > > > > > > same grace period. This guarantee requires a rather big hammer. > > > > > > > > > > > > But yes, let's talk at LPC! > > > > > > > > > > Sounds great, looking forward to discussing this. > > > > > > > > Would it make sense to have an RCU-implementation BoF? > > > > > > > > > > > Also about GP memory ordering and RCU-tree-locking, I think you mentioned to > > > > > > > me that the RCU reader-sections are virtually extended both forward and > > > > > > > backward and whereever it ends, those paths do heavy-weight synchronization > > > > > > > that should be sufficient to prevent memory ordering issues (such as those > > > > > > > you mentioned in the Requierments document). That is exactly why we don't > > > > > > > need explicit barriers during rcu_read_unlock. If I recall I asked you why > > > > > > > those are not needed. So that answer made sense, but then now on going > > > > > > > through the 'Memory Ordering' document, I see that you mentioned there is > > > > > > > reliance on the locking. Is that reliance on locking necessary to maintain > > > > > > > ordering then? > > > > > > > > > > > > There is a "network" of locking augmented by smp_mb__after_unlock_lock() > > > > > > that implements the all-to-all memory ordering mentioned above. But it > > > > > > also needs to handle all the possible complete()/wait_for_completion() > > > > > > races, even those assisted by hypervisor vCPU preemption. > > > > > > > > > > I see, so it sounds like the lock network is just a partial solution. For > > > > > some reason I thought before that complete() was even called on the CPU > > > > > executing the callback, all the CPUs would have acquired and released a lock > > > > > in the "lock network" atleast once thus ensuring the ordering (due to the > > > > > fact that the quiescent state reporting has to travel up the tree starting > > > > > from the leaves), but I think that's not necessarily true so I see your point > > > > > now. > > > > > > > > There is indeed a lock that is unconditionally acquired and released by > > > > wait_for_completion(), but it lacks the smp_mb__after_unlock_lock() that > > > > is required to get full-up any-to-any ordering. And unfortunate timing > > > > (as well as spurious wakeups) allow the interaction to have only normal > > > > lock-release/acquire ordering, which does not suffice in all cases. > > > > > > Sorry to be so persistent, but I did spend some time on this and I still > > > don't get why every CPU would _not_ have executed smp_mb__after_unlock_lock at least > > > once before the wait_for_completion() returns, because every CPU should have > > > atleast called rcu_report_qs_rdp() -> rcu_report_qs_rnp() atleast once to > > > report its QS up the tree right?. Before that procedure, the complete() > > > cannot happen because the complete() itself is in an RCU callback which is > > > executed only once all the QS(s) have been reported. > > > > > > So I still couldn't see how the synchronize_rcu can return without the > > > rcu_report_qs_rnp called atleast once on the CPU reporting its QS during a > > > grace period. > > > > > > Would it be possible to provide a small example showing this in least number > > > of steps? I appreciate your time and it would be really helpful. If you feel > > > its too complicated, then feel free to keep this for LPC discussion :) > > > > The key point is that "at least once" does not suffice, other than for the > > CPU that detects the end of the grace period. The rest of the CPUs must > > do at least -two- full barriers, which could of course be either smp_mb() > > on the one hand or smp_mb__after_unlock_lock() after a lock on the other. > > I thought I'll atleast get an understanding of the "atleast two full > barriers" point and ask you any questions at LPC, because that's what I'm > missing I think. Trying to understand what can go wrong without two full > barriers. I'm sure an RCU implementation BoF could really in this regard. > > I guess its also documented somewhere in Tree-RCU-Memory-Ordering.html but a > quick search through that document didn't show a mention of the two full > barriers need.. I think its also a great idea for us to document it there > and/or discuss it during the conference. > > I went through the litmus test here for some hints on the two-barriers but > couldn't find any: > https://lkml.org/lkml/2017/10/5/636 > > Atleast this commit made me think no extra memory barrier is needed for > tree RCU: :-\ > https://lore.kernel.org/patchwork/patch/813386/ > > I'm sure your last email will be useful to me in the future once I can make > more sense of the ordering and the need for two full barriers, so thanks a > lot for writing it!
Hmmm... I have had this argument before, haven't I? Perhaps I should take some time and get my story straight. ;-)
In my defense, I have been doing RCU since the early 1990s, long before executable formal memory models. I kept it working through sheer paranoia, and that is a hard habit to shake...
Thanx, Paul
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