> On Dec 15, 2022, at 5:10 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Thu, Dec 15, 2022 at 04:42:15PM -0500, Joel Fernandes wrote: >> >> >>>> On Dec 15, 2022, at 4:39 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: >>> >>> On Thu, Dec 15, 2022 at 03:33:39PM -0500, Joel Fernandes wrote: >>>>> On Thu, Dec 15, 2022 at 3:03 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: >>>>> >>>>> Hi Paul, >>>>> >>>>>> On Thu, Dec 15, 2022 at 2:58 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: >>>>> [...] >>>>>>> If the first read section's srcu_read_unlock() and its corresponding >>>>>>> smp_mb() happened before the flip, then the increment of old idx >>>>>>> would happen only once. The next srcu_read_lock() will read the new >>>>>>> index. If the srcu_read_unlock() and it's corresponding smp_mb() >>>>>>> happened after the flip, the old_idx will be sampled again and can be >>>>>>> incremented twice. So it depends on how the flip races with >>>>>>> srcu_read_unlock(). >>>>>> >>>>>> I do understand that a number of people like reasoning about >>>>>> memory-barrier ordering, courtesy of the sequentially consistent portions >>>>>> of the C and C++ memory models, but thinking in terms of the accesses >>>>>> surrounding the memory barriers has been far less error-prone. >>>>> >>>>> Sure, but we are already talking in terms of the access to idx right? >>>>> That's what we're saying is visible by memory barriers and we are >>>>> trying to reason here about the ordering (flip does the write to idx >>>>> and followed by smp_mb(), and there is corresponding read of idx on >>>>> the srcu_read_lock() side. So we are indeed talking in terms of >>>>> access, but let me know if I missed something. >>>>> >>>>>>> Also, since this is all hard to reason about I started making some >>>>>>> diagrams, LOL. For your amusement, here is why need to scan both idx >>>>>>> during grace period detection: https://i.imgur.com/jz4bNKd.png >>>>>> >>>>>> Nice! >>>>>> >>>>>> I suggest placing a gap between GP 2 and GP 3. That way, you can make it >>>>>> very clear that Reader 1's critical section starts after the end of GP 2 >>>>>> (thus clearly never blocking GP 2) and before GP 3 (thus possibly having >>>>>> a reference to some data that is going to be freed at the end of GP 3). >>>>>> >>>>>> I also suggest coloring Reader 1 red and Reader 2 green, given that the >>>>>> color red generally indicates danger. >>>>> >>>>> Thanks for these suggestions! I will make the update. I am planning to >>>>> make a number of diagrams for other scenarios as well, as it helps >>>>> visualize. Google drawing is nice for these. I am happy to share these >>>>> with you all if there is interest :). >>>> >>>> I made these updates, please see: https://i.imgur.com/hoKLvtt.png >>>> >>>> Feel free to use the image for any purpose and thanks ;-) >>> >>> Very good, thank you! >>> >>> Would it be possible to have an arrow marked "X" or "reference to X" >>> from the beginning of the 'Mark "x" for GC' box to the box labeled >>> 'Enter RSCS (access "X")'? >> >> I am currently away from desk. I shared the google drawing with you. Could you check and make the change, if that’s ok with you? >> >> Thank you so much, > > I took a cut at it. Thoughts? Yes perfect now :) and handy future reference! Thanks! - Joel > > Thanx, Paul
