Re: [PATCH] man/man7/epoll.7: Note that an edge-triggered listener needs an accept loop

[Alejandro Colomar <alx@kernel.org>]

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Hi Ben,

> Date: 2026-10-15 21:48:33+0800
> From: Ben Kuo via B4 Relay <devnull+ben.kuo.tw.proton.me@kernel.org>
>
> From: Ben Kuo <ben.kuo.tw@proton.me>
> 
> The example registers the listening socket as level-triggered and
> calls accept(2) once per event, but doesn't say that this only works
> for a level-triggered listener.  If a reader adds EPOLLET to the
> listening socket in the example, connections are left in the accept
> queue and the server stalls.
> 
> Tested on Linux 6.12 (Debian 13, arm64) with an echo server whose
> listening socket is registered with EPOLLET, and a client that opens
> 2000 connections at once.  With one accept4(2) call per listener
> event, the client never finishes: ss(8) shows 1420 connections left
> in the accept queue (Recv-Q), while the server stays blocked in
> epoll_wait(2).  With accept4(2) called in a loop until EAGAIN, all
> connections are echoed.
> 
> Link: <https://github.com/benkuotw/epoll-lab>
> Signed-off-by: Ben Kuo <ben.kuo.tw@proton.me>
> ---

Thanks!  I've applied the patch.


Have a lovely day!
Alex

>  man/man7/epoll.7 | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/man/man7/epoll.7 b/man/man7/epoll.7
> index 2628cbb168b499786e495cf3c614fc5e58217295..aff844a568a5cd6d8eb310ed833266a0ccfd0a4d 100644
> --- a/man/man7/epoll.7
> +++ b/man/man7/epoll.7
> @@ -339,6 +339,16 @@ for (;;) {
>  .EE
>  .in
>  .P
> +The listening socket in this example is level-triggered.
> +If it is registered with
> +.BR EPOLLET ,
> +.BR accept (2)
> +must be called in a loop until it fails with
> +.BR EAGAIN :
> +a single event may report several pending connections,
> +and no further event is reported for those still queued
> +until another connection arrives.
> +.P
>  When used as an edge-triggered interface, for performance reasons, it is
>  possible to add the file descriptor inside the
>  .B epoll
> 
> ---
> base-commit: c6cefe27dd02beaf518bb654493e5f09e62b3225
> change-id: 20261015-epoll-et-listener-c9873cce62bc
> 
> Best regards,
> -- 
> Ben Kuo <ben.kuo.tw@proton.me>
> 
> 
> 

-- 
<https://www.alejandro-colomar.es>

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