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 ;-) - Joel
