On Thu, Dec 15, 2022 at 5:48 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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. > > 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 am sorry this is inverted, but my statement's gist stands I believe: 1. Flip+smp_mb() happened before unlock's smp_mb() -- reader will not increment old_idx the second time. 2. unlock()'s smp_mb() happened before Flip+smp_mb() , now the reader has no new smp_mb() that happens AFTER the flip happened. So it can totally sample the old idx again -- that particular reader will increment twice, but the next time, it will see the flipped one. Did I get that right? Thanks.
