Messages in this thread | | | Date | Wed, 18 Jan 2023 21:30:31 +0100 | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) | From | Jonas Oberhauser <> |
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On 1/18/2023 9:19 PM, Paul E. McKenney wrote: > On Wed, Jan 18, 2023 at 08:42:36PM +0100, Jonas Oberhauser wrote: >> On 1/18/2023 5:50 PM, Alan Stern wrote: >>> On Tue, Jan 17, 2023 at 07:50:41PM -0800, Paul E. McKenney wrote: >>>> On Tue, Jan 17, 2023 at 03:15:06PM -0500, Alan Stern wrote: >>>>> On Tue, Jan 17, 2023 at 09 >>>>>> Given an Srcu-down and an Srcu-up: >>>>>> >>>>>> let srcu-rscs = ( return_value(Srcu-lock) ; (dep | rfi)* ; >>>>>> parameter(Srcu-unlock, 2) ) | >>>>>> ( return_value(Srcu-down) ; (dep | rf)* ; >>>>>> parameter(Srcu-up, 2) ) >>>>>> >>>>>> Seem reasonable, or am I missing yet something else? >>>>> Not at all reasonable. >>>>> >>>>> For one thing, consider this question: Which statements lie inside a >>>>> read-side critical section? >>>> Here srcu_down_read() and srcu_up_read() are to srcu_read_lock() and >>>> srcu_read_unlock() as down_read() and up_read() are to mutex_lock() >>>> and mutex_unlock(). Not that this should be all that much comfort >>>> given that I have no idea how one would go about modeling down_read() >>>> and up_read() in LKMM. >>> It might make sense to work on that first, before trying to do >>> srcu_down_read() and srcu_up_read(). >>> >>>>> With srcu_read_lock() and a matching srcu_read_unlock(), the answer is >>>>> clear: All statements po-between the two. With srcu_down_read() and >>>>> srcu_up_read(), the answer is cloudy in the extreme. >>>> And I agree that it must be clearly specified, and my that previous try >>>> was completely lacking. Here is a second attempt: >>>> >>>> let srcu-rscs = (([Srcu-lock] ; data ; [Srcu-unlock]) & loc) | >>>> (([Srcu-down] ; (data | rf)* ; [Srcu-up]) & loc) >>>> >>>> (And I see your proposal and will try it.) >>>> >>>>> Also, bear in mind that the Fundamental Law of RCU is formulated in >>>>> terms of stores propagating to a critical section's CPU. What are we to >>>>> make of this when a single critical section can belong to more than one >>>>> CPU? >>>> One way of answering this question is by analogy with down() and up() >>>> when used as a cross-task mutex. Another is by mechanically applying >>>> some of current LKMM. Let's start with this second option. >>>> >>>> LKMM works mostly with critical sections, but we also discussed ordering >>>> based on the set of events po-after an srcu_read_lock() on the one hand >>>> and the set of events po-before an srcu_read_unlock() on the other. >>>> Starting here, the critical section is the intersection of these two sets. >>>> >>>> In the case of srcu_down_read() and srcu_up_read(), as you say, whatever >>>> might be a critical section must span processes. So what if instead of >>>> po, we used (say) xbstar? Then given a set of A such that ([Srcu-down ; >>>> xbstar ; A) and B such that (B ; xbstar ; [Srcu-up]), then the critical >>>> section is the intersection of A and B. >>>> >>>> One objection to this approach is that a bunch of unrelated events could >>>> end up being defined as part of the critical section. Except that this >>>> happens already anyway in real critical sections in the Linux kernel. >>>> >>>> So what about down() and up() when used as cross-task mutexes? >>>> These often do have conceptual critical sections that protect some >>>> combination of resource, but these critical sections might span tasks >>>> and/or workqueue handlers. And any reasonable definition of these >>>> critical sections would be just as likely to pull in unrelated accesses as >>>> the above intersection approach for srcu_down_read() and srcu_up_read(). >>>> >>>> But I am just now making all this up, so thoughts? >>> Maybe we don't really need to talk about read-side critical sections at >>> all. Once again, here's what explanation.txt currently says: >>> >>> For any critical section C and any grace period G, at least >>> one of the following statements must hold: >>> >>> (1) C ends before G does, and in addition, every store that >>> propagates to C's CPU before the end of C must propagate to >>> every CPU before G ends. >>> >>> (2) G starts before C does, and in addition, every store that >>> propagates to G's CPU before the start of G must propagate >>> to every CPU before C starts. >>> >>> Suppose we change this to: >>> >>> For any RCU lock operation L and matching unlock operation U, >>> and any matching grace period G, at least one of the following >>> statements must hold: >>> >>> (1) U executes before G ends, and in addition, every store that >>> propagates to U's CPU before U executes must propagate to >>> every CPU before G ends. >>> >>> (2) G starts before L executes, and in addition, every store that >>> propagates to G's CPU before the start of G must propagate >>> to every CPU before L executes. >>> >>> (For SRCU, G matches L and U if it operates on the same srcu structure.) >> I think for the formalization, the definition of "critical section" is >> hidden inside the word "matching" here. >> You will still need to define what matching means for up and down. >> Can I understand down and up to create a large read-side critical section >> that is shared between multiple threads, analogously to a semaphore? With >> the restriction that for srcu, there are really multiple (two) such critical >> sections that can be open in parallel, which are indexed by the return value >> of down/the input of up? >> >> If so I suspect that every down matches with every up within a "critical >> section"? >> maybe you can define balancing along the co analous to the balancing along >> po currently used for matching rcu_lock() and rcu_unlock(). I.e., >> >> down ------------- up >> \down--------up/ >> \down-up/ >> \_/ >> where diagonal links are co links and the straight links are "balanced >> match" links. > The SRCU read-side critical sections are fundamentally different than > those of RCU. [...] > In contrast, SRCU read-side critical sections are defined by the > return value of srcu_read_lock() being passed into the matching > srcu_read_unlock().
I'm a bit confused. I previously thought that there is srcu_lock/srcu_unlock and srcu_down/srcu_up and that these are different things.
Your explanation matches how I understood srcu_read_lock after reading the paper and pretending that there wasn't a single counter, while my understanding of srcu_read_down would be closer to the original implementation in the 2009 paper where there was a single counter, and thus srcu_read_down and srcu_read_up could open a multi-thread critical section.
Is there only one thing and read_down *is* read_lock? If they are not the same, is my understand of read_down correct?
And isn't it also true that the srcu_lock() needs to be on the same CPU as the matching srcu_unlock()?
I think for matching srcu_lock to srcu_unlock, you can just use the data dependency (following the "hack" of defining them as reads and writes).
What I was suggesting below is how to redefine "match" between read_down and read_up that work more like a cross-thread semaphore.
>> Then everything that is enclosed within a pair of "balanced match" is >> linked: >> >> match-down-up = co^-1?; balanced-srcu-updown ; co^-1? >> >> Since multiple critical sections can be in-flight, maybe you can use co & >> same-value (or whatever the relation is) to define this? >> >> >> let balanced-srcu-updown = let rec >> unmatched-locks = Srcu-down \ domain(matched) >> and unmatched-unlocks = Srcu-up \ range(matched) >> and unmatched = unmatched-locks | unmatched-unlocks >> and unmatched-co = [unmatched] ; co & same-value ; [unmatched] >> and unmatched-locks-to-unlocks = >> [unmatched-locks] ; co & same-value ; [unmatched-unlocks] >> and matched = matched | (unmatched-locks-to-unlocks \ >> (unmatched-co ; unmatched-co)) >> in matched >> let match-down-up = (co & same-value)^-1?; balanced-srcu-updown ; (co & >> same-value)^-1?
>> Is the implementation of srcu-lock and srcu-unlock still anything like the >> implementation in the 2009 paper? > The interaction between readers and grace period is now mediated by a > per-CPU pair of lock counters and of unlock counters, so the 2009 paper is > not the best guide. But yes, you would likely need three or four pairwise > overlapping critical sections for the current SRCU implementation to end > "early".
That makes sense.
Have fun, jonas
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