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SubjectRe: [PATCH RFC] srcu: Yet more detail for srcu_readers_active_idx_check() comments
Hi,
On the related subject of this function, I drew a diagram for one of
the reasons why per-CPU unlock counts have to be scanned first, for a
particular index, before the per-CPU lock counts, and not the other
way. Otherwise, a reader that got preempted after reading the index,
can suddenly get scheduled during the inactive index's scan, and cause
the total lock and unlock counts to falsely match:
https://i.imgur.com/79fDWdQ.png

Cheers,

- Joel



On Fri, Dec 16, 2022 at 11:54 AM Joel Fernandes <joel@joelfernandes.org> wrote:
>
>
>
> > On Dec 16, 2022, at 11:51 AM, Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Fri, Dec 16, 2022 at 04:32:39PM +0000, Joel Fernandes wrote:
> >> On Thu, Dec 15, 2022 at 05:09:14PM -0800, Paul E. McKenney wrote:
> >> [...]
> >>>>>> 2. unlock()'s smp_mb() happened before Flip+smp_mb() , now the reader
> >>>>>> has no new smp_mb() that happens AFTER the flip happened. So it can
> >>>>>> totally sample the old idx again -- that particular reader will
> >>>>>> increment twice, but the next time, it will see the flipped one.
> >>>>>
> >>>>> I will let you transliterate both. ;-)
> >>>>
> >>>> I think I see what you mean now :)
> >>>>
> >>>> I believe the access I am referring to is the read of idx on one side and
> >>>> the write to idx on the other. However that is incomplete and I need to
> >>>> pair that with some of other access on both sides.
> >>>>
> >>>> So perhaps this:
> >>>>
> >>>> Writer does flip + smp_mb + read unlock counts [1]
> >>>>
> >>>> Reader does:
> >>>> read idx + smp_mb() + increment lock counts [2]
> >>>>
> >>>> And subsequently reader does
> >>>> Smp_mb() + increment unlock count. [3]
> >>>>
> >>>> So [1] races with either [2] or [2]+[3].
> >>>>
> >>>> Is that fair?
> >>>
> >>> That does look much better, thank you!
> >>
> >> Perhaps a comment with an ASCII diagram will help?
> >>
> >>
> >> Case 2:
> >> Both the reader and the updater see each other's writes too late, but because
> >> of memory barriers on both sides, they will eventually see each other's write
> >> with respect to their own. This is similar to the store-buffer problem. This
> >> let's a single reader contribute a maximum (unlock minus lock) imbalance of 2.
> >>
> >> The following diagram shows the subtle worst case followed by a simplified
> >> store-buffer explanation.
> >>
> >> READER UPDATER
> >> ------------- ----------
> >> // idx is initially 0.
> >> read_lock() {
> >> READ(idx) = 0;
> >> lock[0]++; --------------------------------------------,
> >> flip() { |
> >> smp_mb(); |
> >> smp_mb(); |
> >> } |
> >> |
> >> // RSCS |
> >> |
> >> read_unlock() { |
> >> smp_mb(); |
> >> idx++; // P |
> >> smp_mb(); |
> >> } |
> >> |
> >> scan_readers_idx(0) { |
> >> count all unlock[0]; |
> >> | |
> >> | |
> >> unlock[0]++; //X <--not-counted--`-----, |
> >> | |
> >> } V `------,
> >> // Will make sure next scan |
> >> // will not miss this unlock (X) |
> >> // if other side saw flip (P) ,--`
> >> // Call this MB [1] |
> >> // Order write(idx) with |
> >> // next scan's unlock. |
> >> smp_mb(); ,---`
> >> read_lock() { |
> >> READ(idx)=0; |
> >> lock[0]++; ----------------> count all lock[0]; |
> >> smp_mb(); | } |
> >> } | | V
> >> | `---> // Incorrect contribution to lock counting
> >> | // upto a maximum of 2 times.
> >> |
> >> `---> // Pairs with MB [1]. Makes sure that
> >> // the next read_lock()'s' idx read (Y) is ordered
> >> // with above write to unlock[0] (X).
> >> |
> >> rcu_read_unlock() { |
> >> smp_mb(); <---------------`
> >> unlock[0]++;
> >> }
> >>
> >> read_lock() {
> >> READ(idx) = 1; //Y
> >> lock[1]++;
> >> ...
> >> }
> >> scan_readers_idx(0) {
> >> count all unlock[0]; //Q
> >> ...
> >>
> >>
> >> thanks,
> >>
> >> - Joel
> >>
> >> }
> >>
> >> This makes it similar to the store buffer pattern. Using X, Y, P and Q
> >> annotated above, we get:
> >>
> >> READER UPDATER
> >> X (write) P (write)
> >>
> >> smp_mb(); smp_mb();
> >>
> >> Y (read) Q (read)
> >
> > Given that this diagram is more than 50 lines long, it might go better in
> > a design document describing this part of RCU. Perhaps less detail or
> > segmented, but the same general idea as this guy:
> >
> > Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
>
> Yes, this sounds like a good place to add it and perhaps we refer to it from the C source file? I can take this up to do over the holidays, if you prefer.
>
> Thanks,
>
> - Joel
>
>
> >
> > Thoughts?
> >
> > Thanx, Paul

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