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