Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Paul E. McKenney
2023-01-18T21:13:05Z

On Wed, Jan 18, 2023 at 09:30:31PM +0100, Jonas Oberhauser wrote:
> 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).

The only difference between srcu_read_lock() and srcu_read_unlock()
on the one hand and srcu_down_read() and srcu_up_read() on the other
is that a matching pair of srcu_read_lock() and srcu_read_unlock()
must be running on the same task. In contrast, srcu_down_read() and
srcu_up_read() are not subject to this constraint.

> What I was suggesting below is how to redefine "match" between read_down and
> read_up that work more like a cross-thread semaphore.

Understood, but what I don't understand is why not simply this:

let srcu-rscs-down = ([Srcu-down] ; (data | rf)* ; [Srcu-up]) & loc

> > > 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

And you! ;-)

Thanx, Paul