Re: Regression: epoll edge-triggered (EPOLLET) for pipes/FIFOs

[Linus Torvalds]

On Mon, Oct 12, 2020 at 1:30 PM Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> wrote:
>
> [CC += Davide]

I'm not sure how active Davide is any more..

> I don't think this is correct. The epoll(7) manual page
> sill carries the text written long ago by Davide Libenzi,
> the creator of epoll:
>
>     Since  even with edge-triggered epoll, multiple events can be gen‐
>     erated upon receipt of multiple chunks of data, the caller has the
>     option  to specify the EPOLLONESHOT flag, to tell epoll to disable
>     the associated file descriptor after the receipt of an event  with
>     epoll_wait(2).
>
> My reading of that text is that in the scenario that I describe a
> readiness notification should be generated at step 3 (and indeed
> should be generated whenever additional data bleeds into the channel).

Hmm.

That is unfortunate, because it basically exposes an internal wait
queue implementation decision, not actual real semantics.

I suspect it's easy enough to "fix" the regression with the attached
patch. It's pretty nonsensical, but I guess there's not a lot of
downside - if the pipe wasn't empty, there normally shouldn't be any
non-epoll readers anyway.

I'm busy merging, mind testing this odd patch out? It is _entirely_
untested, but from the symptoms I think it's the obvious fix.

I did the same thing for the "reader starting out from a full pipe" case too.

               Linus

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