michallenc commented on PR #21477:
URL: https://github.com/apache/nuttx/pull/21477#issuecomment-6681518225
> > There seems to be a change of behavior compared to the previous version
as this patch breaks the following code we use in Modbus application:
> > ```c
> > int epollfd = epoll_create1(0);
> >
> > struct eserver eservers[state->nservers];
> > for (int i = 0; i < state->nservers; i++) {
> > eservers[i] = (struct eserver){
> > .type = EWAITER_SERVER,
> > .server = state->servers[i],
> > };
> > struct epoll_event ev = {
> > .events = EPOLLIN | EPOLLHUP,
> > .data.ptr = &eservers[i],
> > };
> > int ret = epoll_ctl(epollfd, EPOLL_CTL_ADD,
> > modbus_server_pollfd(state->servers[i]), &ev);
> > assert(ret == 0);
> > }
> >
> > while (!halt || !*halt) {
> > struct epoll_event rev;
> > int ret = epoll_wait(epollfd, &rev, 1, timeout);
> > ...
> > ```
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > doesn't return at all. The fd obtained by `modbus_server_pollfd` is a
TCP socket.
> > If the same code is run with serial server (fd for `epoll` is created by
`eventfd(0,0)` and then `eventfd_write(s->efd, 42)` is called), then the
`epoll_wait` returns multiple times, not just once as before the patch.
>
> @michallenc Thanks for the report. I fixed the behavior regression.
>
> The issue was that rearming a delivered level-triggered node could consume
the synchronous readiness notification generated during the driver poll setup
and enqueue the node again immediately. This made eventfd report repeatedly
after a single eventfd_write(), and could also affect TCP socket rearming.
>
> I updated the epoll rearm path to suppress the synchronous setup-time
notification while keeping the pollfd armed for future lower-half
notifications. So a delivered node is rearmed, but it will not be reported
again until a new driver notification occurs.
Thank you. I won't be able to test until Monday morning though, hope that's
ok.
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