Re: [PATCH RFC] srcu: Yet more detail for srcu_readers_active_idx_check() comments

[Paul E. McKenney]

On Fri, Dec 16, 2022 at 11:54:19AM -0500, Joel Fernandes wrote:
> 
> 
> > On Dec 16, 2022, at 11:51 AM, Paul E. McKenney <paulmck@xxxxxxxxxx> 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.

Indeed, that comment is quite large already, arguably obscuring the code!
It would be good to offload some of it.

							Thanx, Paul