Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Alan Stern
2023-01-20T16:01:12Z

On Wed, Jan 18, 2023 at 04:02:14PM -0800, Paul E. McKenney wrote:
> There are pairs of per-CPU counters. One pair (->srcu_lock_count[])
> counts the number of srcu_down_read() operations that took place on
> that CPU and another pair (->srcu_unlock_count[]) counts the number
> of srcu_down_read() operations that took place on that CPU. There is
> an ->srcu_idx that selects which of the ->srcu_lock_count[] elements
> should be incremented by srcu_down_read(). Of course, srcu_down_read()
> returns the value of ->srcu_idx that it used so that the matching
> srcu_up_read() will use that same index when incrementing its CPU's
> ->srcu_unlock_count[].
>
> Grace periods go something like this:
>
> 1. Sum up the ->srcu_unlock_count[!ssp->srcu_idx] counters.
>
> 2. smp_mb().
>
> 3. Sum up the ->srcu_unlock_count[!ssp->srcu_idx] counters.

Presumably you meant to write "lock" here, not "unlock".

>
> 4. If the sums are not equal, retry from #1.
>
> 5. smp_mb().
>
> 6. WRITE_ONCE(ssp->srcu_idx, !ssp->srcu_idx);
>
> 7. smp_mb().
>
> 8. Same loop as #1-4.
>
> So similar to r/w semaphores, but with two separate distributed counts.
> This means that the number of readers need not go to zero at any given
> point in time, consistent with the need to wait only on old readers.

Reasoning from first principles, I deduce the following:

You didn't describe exactly how srcu_down_read() and srcu_up_read()
work. Evidently the unlock increment in srcu_up_read() should have
release semantics, to prevent accesses from escaping out the end of the
critical section. But the lock increment in srcu_down_read() has to be
stronger than an acquire; to prevent accesses in the critical section
escaping out the start, the increment has to be followed by smp_mb().

The smp_mb() fences in steps 5 and 7 appear to be completely
unnecessary.

Provided an smp_mb() is added at the very start and end of the grace
period, the memory barrier in step 2 and its copy in step 8 can be
demoted to smp_rmb().

These changes would be small optimizations at best, and you may consider
them unimportant in view of the fact that grace periods often last quite
a long time.

Alan