Hello,
On Fri, Oct 30, 2026 at 11:50:36PM +0100, Nora Sundman wrote:
> BUG/MINOR: ring: fix lost reader wakeup on a message being written
>
> Since commit cb482f92c4 ("MINOR: ring: make sure ring_dispatch waits when
> facing a changing message"), a reader's pass stops on a message whose
> writer is still copying it. The tail already covers that message, so the
> check made after registering in ring->waiters (did the tail move since the
> pass started ?) cannot see it complete. If the writer completes and wakes
> the waiters between the end of the pass and the registration, the wakeup
> is lost and the reader sleeps with a complete message in the ring until
> another one is written.
>
> For the log forwarder ("log tcp@", log backends, ring sections with
> servers) and "show events -w", this means that lines stay unsent until the
> next message, possibly for a long time on a quiet ring, and are lost if the
> process dies meanwhile. It was seen in 3 bursts out of 30000 on an
> unmodified build, and can be reproduced by delaying a writer and a reader.
>
> Let's instead check once registered whether a complete message is
> available at the reader's offset, using the new ring_msg_ready(). A barrier
> orders the registration before this check, which is free on x86. The tail
> comparison being replaced, last_ofs_ptr is now unused and is removed from
> ring_dispatch_messages().
>
> This must be backported to 3.0 and above.
Interesting, thank you! I'll rerun some perf tests before merging it, as
this part is quite sensitive, to make sure we don't overlook anything,
but generally speaking the description looks good.
Thanks!
Willy
