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

Paul E. McKenney
2023-01-26T01:53:35Z

On Wed, Jan 25, 2023 at 08:45:44PM -0500, Alan Stern wrote:
> On Wed, Jan 25, 2023 at 03:33:08PM -0800, Paul E. McKenney wrote:
> > Ah, and returning to the earlier question as to whether srcu_read_unlock()
> > can use release semantics instead of smp_mb(), at the very least, this
> > portion of the synchronize_srcu() function's header comment must change:
> >
> > On systems with more than one CPU, when synchronize_srcu()
> > returns, each CPU is guaranteed to have executed a full
> > memory barrier since the end of its last corresponding SRCU
> > read-side critical section whose beginning preceded the call
> > to synchronize_srcu().
>
> Yes, that would not be true. But on the other hand, it would be true
> that each CPU is guaranteed to have executed a release memory barrier
> since the end of its last corresponding SRCU read-side critical section
> whose beginning preceded the call to synchronize_srcu(), _and_ the CPU
> executing synchronize_srcu() is guaranteed to have executed a full
> memory barrier after seeing the values from all those release stores.
> This is not quite the same thing but it ought to be just as good.

Here is hoping!

> > I don't know of any SRCU code that relies on this, but it would be good to
> > check. There used to (and might still) be RCU code relying on this, which
> > is why this sentence was added to the header comment in the first place.
>
> If there is code relying on that guarantee, it ought to work just as
> well by relying on the modified guarantee.

Again, here is hoping!

> Of course, there might be code relying on a guarantee that
> srcu_read_unlock() executes a full memory barrier. This guarantee would
> certainly no longer hold. But as I understand it, this guarantee was
> never promised by the SRCU subsystem.

That indented sentence was copied from the synchronize_srcu() function's
header comment, which might be interpreted by some as a promise by the
SRCU subsystem.

Thanx, Paul