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. 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(). 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 thanks, - Joel
