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

Paul E. McKenney
2023-01-20T17:58:53Z

On Fri, Jan 20, 2023 at 11:01:03AM -0500, Alan Stern wrote:
> 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".

You are quite right, and apologies for my confusion.

> > 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().

You got it! There is some work going on to see if srcu_read_lock()'s
smp_mb() can be weakened to pure release, but we will see.

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

For correctness, agreed. Their purpose is instead forward progress.
One can argue that step 5 is redundant due to control dependency, but
control dependencies are fragile, and as you say below, this code is
nowhere near a fastpath.

> 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().

This might need to be smp_mb() to allow srcu_read_unlock() to be
demoted to release ordering. Work in progress.

> 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.

Agreed, keeping it simple and obvious is important on this code, which
is nowhere near a fastpath. The case of srcu_read_unlock() is another
thing altogether.

Thanx, Paul