On Thu, Dec 15, 2022 at 05:58:14PM +0000, Joel Fernandes wrote: > 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. By "increment old_idx" you mean "increment ->srcu_lock_count[old_idx]", correct? Again, the important ordering isn't the smp_mb(), but the accesses, in this case, the accesses to ->srcu_unlock_count[idx]. > 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. I will let you transliterate both. ;-) > Did I get that right? Thanks. So why am I unhappy with orderings of smp_mb()? To see this, let's take the usual store-buffering litmus test: CPU 0 CPU 1 WRITE_ONCE(x, 1); WRITE_ONCE(y, 1); smp_mb(); smp_mb(); r0 = READ_ONCE(y); r1 = READ_ONCE(x); Suppose CPU 0's smp_mb() happens before that of CPU 1: CPU 0 CPU 1 WRITE_ONCE(x, 1); WRITE_ONCE(y, 1); smp_mb(); smp_mb(); r0 = READ_ONCE(y); r1 = READ_ONCE(x); We get r0 == r1 == 1. Compare this to CPU 1's smp_mb() happening before that of CPU 0: CPU 0 CPU 1 WRITE_ONCE(x, 1); WRITE_ONCE(y, 1); smp_mb(); smp_mb(); r0 = READ_ONCE(y); r1 = READ_ONCE(x); We still get r0 == r1 == 1. Reversing the order of the two smp_mb() calls changed nothing. But, if we order CPU 1's write to follow CPU 0's read, then we have this: CPU 0 CPU 1 WRITE_ONCE(x, 1); smp_mb(); r0 = READ_ONCE(y); WRITE_ONCE(y, 1); smp_mb(); r1 = READ_ONCE(x); Here, given that r0 had the final value of zero, we know that r1 must have a final value of 1. And suppose we reverse this: CPU 0 CPU 1 WRITE_ONCE(y, 1); smp_mb(); r1 = READ_ONCE(x); WRITE_ONCE(x, 1); smp_mb(); r0 = READ_ONCE(y); Now there is a software-visible difference in behavior. The value of r0 is now 1 instead of zero and the value of r1 is now 0 instead of 1. Does this make sense? Thanx, Paul
