Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

Frederic Weisbecker
2022-12-21T00:50:08Z

On Tue, Dec 20, 2022 at 07:15:00PM -0500, Joel Fernandes wrote:
> On Tue, Dec 20, 2022 at 5:45 PM Frederic Weisbecker <frederic@kernel.org> wrote:
> Agreed about (1).
>
> > _ In (2), E pairs with the address-dependency between idx and lock_count.
>
> But that is not the only reason. If that was the only reason for (2),
> then there is an smp_mb() just before the next-scan post-flip before
> the lock counts are read.

The post-flip barrier makes sure the new idx is visible on the next READER's
turn, but it doesn't protect against the fact that "READ idx then WRITE lock[idx]"
may appear unordered from the update side POV if there is no barrier between the
scan and the flip.

If you remove the smp_mb() from the litmus test I sent, things explode.