> 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, - Joel > > Thanx, Paul
