On Thu, Dec 15, 2022 at 05:48:46PM +0000, Joel Fernandes wrote: > On Thu, Dec 15, 2022 at 5:08 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > Scenario for the reader to increment the old idx once: > > > > > > _ Assume ssp->srcu_idx is initially 0. > > > _ The READER reads idx that is 0 > > > _ The updater runs and flips the idx that is now 1 > > > _ The reader resumes with 0 as an index but on the next srcu_read_lock() > > > it will see the new idx which is 1 > > > > > > What could be the scenario for it to increment the old idx twice? > > > > Unless I am missing something, the reader must reference the > > srcu_unlock_count[old_idx] and then do smp_mb() before it will be > > absolutely guaranteed of seeing the new value of ->srcu_idx. > > I think both of you are right depending on how the flip raced with the > first reader's unlock in that specific task. There are indeed a variety of scenarios and also a variety of failure cases. > 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. > 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. Thanx, Paul
